Method and system for maintaining session continuity when changes occur at the terminal during a session

ABSTRACT

A method and a system for maintaining session continuity, in which the system includes a Handover Source Function (HOSF), a Handover Destination Function (HODF), a Handover Anchor Function-Control Plane (HOAF-CP) and a Handover Anchor Function-User Plane (HOAF-UP). The method includes a first user establishing a connection through the HOSF with the HOAF-CP and the HOAF-UP which correspond to a second user, so as to establish a session with the second user. The method further includes that, during session handover, the HODF that corresponds to the HOSF establishing another connection with the HOAF-CP and HOAF-UP though which the first user continues the session with the second user, so as to maintain the session continuity. When access address or access technology of the terminal changes, or even the terminal changes during a session, the method and the system can logically replace the connection before the handover with a new connection to ensure the session continuity.

CROSS-REFERENCES

This application is a Reissue Application of patent application Ser. No.11/649,022 filed on Dec. 19, 2005, issued as U.S. Pat. No. 8,625,527 B2on Jan. 7, 2014, which is a continuation of International ApplicationNo. PCT/CN2005/002240, filed on Dec. 19, 2005, which. The internationalapplication claims priority to Chinese Patent Application No.200410101259.1, filed on Dec. 17, 2004, Chinese Patent Application No.200410082358.X, filed on Dec. 31, 2004, Chinese Patent Application No.200510033578.8, filed on Mar. 8, 2005, Chinese Patent Application No.200510063705.9, filed on Mar. 30, 2005, and Chinese Patent ApplicationNo. 200510035265.6, filed on Jun. 10, 2005, both of which are herebyincorporated by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates to communication technologies, moreparticularly to a method and a system for maintaining session continuitywhen a user changes access address or access technology of a terminal,or even changes the terminal in a session.

BACKGROUND OF THE INVENTION

As communication and information technology rockets now, and along withthe maturation of Internet Protocol (IP) technology, the application ofthe Internet spreads rapidly. People are no longer satisfied withmonotonous voice communications. Instead, they demand brand newmultimedia communications. At the same time, mobile communicationnetworks and fixed communication networks still serve as the maincommunication medium. Therefore, it is acknowledged in the industry thatit is the trend that the mobile communication network and the fixedcommunication network will develop towards IP and the Internet willmerge with the telecommunication network. In other words, an IPmultimedia communication system which supports multiple accesstechnologies is a focus of the future development in the industry. An IPMultimedia Subsystem (IMS) and a Next Generation Network (NGN) are justthe kind of networks which support multiple access technologies whileimplementing IP multimedia applications.

The IMS is a network subsystem introduced by the 3^(rd) GenerationPartnership Project (3GPP) on the basis of a Universal MobileTelecommunications System (UMTS) Packet Switched (PS) domain to achieveIP multimedia communication control. The IMS supports multiple accesstechnologies, shields user access technology, controls the openness ofservice capability and provides IP multimedia communication experiencebased on customized user data. The IMS employs the UMTS PS domain orother IP access networks as a bearer for upper layer control signalingand media transmission. Major functional entities in the IMS include: aCall Session Control Function (CSCF) for controlling user registrationsand sessions, an Application Server (AS) for providing various servicelogic control functions, a Home Subscriber Server (HSS) for centralizedadministration of subscription data, and a Multimedia Gateway ControlFunction/IM Multimedia Gateway Function (MGCF/IM-MGW) for interworkingwith a Circuit Switched (CS) network. In the IMS, a user accesses theIMS through a Proxy-CSCF (P-CSCF), which is a proxy node in a visitednetwork where the user is currently located. While the session andservice triggering control and the service control interaction with theAS are performed by a Service-CSCF (S-CSCF), which is a service node ina home network of the user.

The IMS provides IMS users with IP multimedia value-added servicesthrough various ASs. In the IMS, services are triggered based on aninitial Filter Criteria (iFC) evaluation. The iFC is a major part of theuser subscription data saved in the HSS and downloaded by the S-CSCEassigned to the user during registration. The iFCs with differentpriority levels defines different service triggering conditions andcorresponding destination ASs. The S-CSCF compares a service requestreceived from or sent to a user it serves with the service triggeringconditions in the iFCs one by one according to their priorities. If theservice request matches the service triggering condition in an iFC, theservice request will be sent to the corresponding AS defined by thisiFC. If the service request does not match the service triggeringconditions in the iFCs, the S-CSCF continues the evaluation of the nextiFC in the descending priority sequence. Or, if the service requestmatches the service triggering condition in an iFC and is sent to thecorresponding AS and the AS returns the service request to the S-CSCF,the S-CSCF also continues the evaluation of the next iFC in thedescending priority sequence. After finishing the evaluation of all theiFCs, the S-CSCF sends the service request to a next network nodeaccording to a destination identifier in the service request. In thismethod, services are triggered according to uniform filter criteriawhich are independent with specific service. Thus the processing modelemployed by the IMS service node S-CSCF can be shared and reused byvarious services, so that the IMS architecture is able to provide richand various customized services through various ASs.

The NGN is a convergent network based on packet technologies, mainlyadopting packet switched technologies and employing a bearer/controlseparated architecture. The NGN inherits all the services of theprevious fixed network as well as some of the mobile network. The NGNintegrates advantages of the fixed network, the mobile network and theIP network so as to allow users, such as analog users, digital users,mobile users, Asymmetrical Digital Subscriber Line (ADSL) users,Integrated Service Digital Network (ISDN) users, IP narrow-band networkusers, IP broadband network users and even satellite users, tocommunicate with each other in the NGN.

Both the IMS and the NGN adopt a Session Initiation Protocol (SIP) astheir session control protocols. The SIP is one of the multimediacommunication system frame protocols defined by Internet EngineeringTask Force (IETF), and is an application layer protocol for theestablishment, modification and termination of a multimedia session.Since the SIP is based on published Internet standards and is innatelyadvantageous with respect to the combination and interworking of a voiceservice and a data service, it can implement session controls acrossmedia and equipment and support varieties of media formats. It should benoted that the SIP supports interaction between two participants of asession to exchange Session Description Protocol (SDP) descriptions ofthe media streams desired to be exchanged in an offer/answer mode duringa session establishment, so as to accomplish a negotiation of theexchanged media streams in the session. In an established session, aparticipant can also exchange the SDP description to change the mediastream desires to be exchanged by re-negotiation, i.e., using theoffer/answer mode again, to dynamically add/delete media streams ormodify the attribute of the exchanged media streams. In this way, it iseasier to present abundant multimedia service features. At the sametime, the SIP supports pushing the intelligence towards applications andterminals and so as to relieve the burden of the network. The SIP alsosupports application layer mobility capability including dynamicregistration, location management, and re-direction mechanisms, as wellas the Presence capability (determining the specific communication meansaccording to the location and status of the user), Fork capability (anability to perform serial attempt or parallel forwarding a servicerequest to multiple valid contact address registered to a destinationidentifier), and Subscribe and Notify mechanism, which makes itfacilitated for the deployment of new services. Moreover, the SIP is asimple protocol with recognized extension potential and is thus widelyapplied in networks including the IMS and the NGN.

In the IMS and the NGN, a user can access the network through amulti-mode terminal, or through different terminals, via access networksof different access technologies, and obtain uniform multimedia servicesaccording to his/her subscription.

In the IMS and the NGN, a user binds his/her current terminal contactaddress and service identifier (which is the user's IP Multimedia Publicidentity (IMPU) in the IMS) through registration. Therefore, during aregistration period, there should be a unique access addresscorresponding to the service identifier of the user. In order words,once the access address changes, the user needs a re-registration, andthe original access address will be deleted, and the on-going sessionwill be released if no special processing is performed in the servicelayer. Therefore, when a user in a session needs to change his/herAccess Point (AP) or access technology which further result in thechange of the access address, or even change the terminal, the problemof maintaining session continuity emerges.

On the other hand, a voice call to a peer user (e.g., a subscriber ofthe IMS or the NGN) can be either established as a Voice over IP (VoIP)call using an end-to-end IP bearer under the control of the IMS/NGN, ordirectly established as an interworking call between traditional CSdomain and the IMS/NGN. Some access technologies through which theterminal accesses the IMS or the NGN belong to hot spot coveragetechnologies (e.g., Wireless Local Access Network (WLAN)). When a useraccesses IMS/NGN through the WLAN to initiate a VoIP call, if the userroams out of the hot spot coverage of the WLAN, the network connectionwill be lost. At this time, if the user can change to access the CSdomain through Wideband Code Division Multiple Access (WCDMA) whichusually implemented with wide, continuous coverage to continue thesession with the peer user, the session continuity will be well kept.Similarly, a WCDMA system, which is designed to cover wide areascontinuously, may fail to provide high quality wireless connection insome buildings due to penetration loss of wireless signals, while hotspot coverage technologies, represented by the WLAN, are able to fillthe coverage holes emerged here. In other words, if the user changes aCS voice call to an IMS VoIP session through the WLAN access when hemoves into such a coverage hole of WCDMA, the session continuity will bewell kept. Considering that the call between the user and the peerIMS/NGN user through the CS domain actually includes a CS callconnection from the user to an interworking gateway between the CSdomain and the IMS/NGN, and a SIP session from the interworking gatewaybetween the CS domain and the IMS/NGN to the called IMS/NGN user. A CScall, which is converted to an IMS/NGN session through the interworkinggateway, can be regarded as a special access means to the IMS/NGN fromthe point of view of the IMS/NGN. So the problem of maintaining sessioncontinuity when a terminal changes access means in an IMS/NGN sessionshould also be taken into consideration to better guarantee the sessioncontinuity.

There exists a possible solution to guarantee the session continuity,i.e., Unlicensed Mobile Access (UMA).

The UMA standards are a set of published specifications jointlydeveloped by a number of leading operators and vendors within thewireless industry. The UMA standards enable a terminal to access acellular network and acquire services of the cellular network throughunlicensed spectrum access technologies such as WLAN and Bluetooth (BT),and eventually enable a dual-mode terminal to roam and handover betweenthe cellular network and the unlicensed spectrum wireless network.

FIG. 1 is a schematic diagram illustrating a process of maintainingsession continuity through the UMA standards according to the prior art.As shown in FIG. 1, a UMA network controller (UNC), whose networklocation is similar to that of a Base Station Controller (BSC) or aRadio Network Controller (RNC) in the cellular network, is added inadvance into the unlicensed spectrum wireless network, e.g., the WLAN.The added UNC is used for providing an interface with the core networkjust like that provided by the BSC/RNC in cellular network. Thereforethe core network may regard the unlicensed spectrum wireless network asa normal cellular access network. When a terminal roams and handoversbetween the cellular network and the unlicensed spectrum wirelessnetwork, handover between the two networks are implemented through theinteraction among the core network, the UNC in the unlicensed spectrumwireless networks and a corresponding entity, such as the BSC, in thecellular network.

SUMMARY OF THE INVENTION

The present invention provides a method and a system for maintainingsession continuity, when access address of a terminal is changed as aresult of alteration of Access Point (AP), access technology, accessmeans or a terminal used by a user, the method and system provided bythe present invention may logically replace an original sessionconnection established before a handover with a new session connectionestablished after the handover to guarantee the session continuity.

The present invention provides the following technical schemes:

A method for maintaining session continuity, wherein there exists acorrelated relationship between a Handover Source Function (HOSF) and aHandover Destination Function (HODF) of a first user, including:

Step A: the first user establishing a session with a second user via afirst connection through the HOSF, including a control plane firstconnection with a Handover Anchor Function-Control Plane (HOAF-CP) and auser plane first connection with a Handover Anchor Function-User Plane(HOAF-UP);

Step B: when it is detected that a handover condition is satisfied, theHODF that corresponds to the HOSF establishing a second connection,including a control plane second connection and a user plane secondconnection, respectively with the HOAF-CP and the HOAD-UP;

Step C: the first user continuing the session with the second userthrough the second connection.

A system for maintaining session continuity includes: an HOSF, an HODF,an HOAF-CP, an HOAF-UP and a Handover Detection Function; wherein,

there exists a corresponding relationship between the HOSF and the HODF;and

the HOSF is used for establishing a session control plane firstconnection with the HOAF-CP and a session user plane first connectionwith the HOAF-UP when the first user initiates or accepts a session;

the HODF is used for establishing a session control plane secondconnection with the HOAF-CP and a session user plane second connectionwith the HOAF-UP when the handover condition is satisfied;

the HOAF-CP is used for establishing the control plane first connectionwith the HOSF when the first user initiates or accepts a session;establishing the control plane second connection with the HODF when thehandover condition is satisfied; replacing the control plane firstconnection with the HOSF with the control plane second connection withthe HODF, and indicating the HOAF-UP to replace the user plane firstconnection between the HOAF-UP and the HOSF with the user plane secondconnection between the HOAF-UP and the HODF;

the HOAF-UP is used for establishing the user plane first connectionwith the HOSF when the first user initiates or accepts a session;establishing the user plane second connection with the HODF when thehandover condition is satisfied; and replacing the user plane firstconnection with the HOSF with the user plane second connection with theHODF according to the indication of the HOAF-CP;

the Handover Detection Function is used for detecting whether thehandover condition is satisfied.

The present invention provides a method and a system on the servicelayer for maintaining session continuity in a network that supportsmultiple access technologies. The method, the two connections before andafter the handover are swapped logically on the service layer while theconnection after the handover is established, therefore the sessionusability after the handover is guaranteed, the session continuity iskept and the service experience of users is enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a procedure of maintainingsession continuity through UMA standards according to the prior art;

FIG. 2A is a schematic diagram illustrating a system when performing asession handover in a terminal-controlled mode according to anembodiment of the present invention;

FIG. 2B is a schematic diagram illustrating a system when performing asession handover in a network-controlled mode according to an embodimentof the present invention;

FIG. 3A is a block diagram logically illustrating a system whenperforming a session handover in an end-to-end/terminal-controlled modeaccording to an embodiment of the present invention;

FIG. 3B is a block diagram logically illustrating a system whenperforming a session handover in an end-to-end/network-controlled modeaccording to an embodiment of the present invention;

FIG. 3C1 is a block diagram logically illustrating a system whenperforming a session handover in a CP-segmented/terminal-controlled modeor a CP-segmented/network-controlled mode according to an embodiment ofthe present invention;

FIG. 3C2 is a block diagram logically illustrating a system whenperforming a session handover in a CPandUP-segmented/terminal-controlledmode or a CPandUP-segmented/network-controlled mode according to anembodiment of the present invention;

FIG. 4 is a schematic diagram illustrating connection relationships whenan IMS session is transferred to a CS call;

FIG. 5A is a flowchart illustrating a handover in anend-to-end/terminal-controlled mode according to an embodiment of thepresent invention, while the HODF initiates the performing of thehandover;

FIG. 5B1 is a flowchart illustrating a handover according to thestructure of the FIG. 3B;

FIG. 5B2 is a flowchart illustrating a procedure of the handover UEestablishing a new connection for the handover under the control of aservice control point based on the structure of FIG. 3B according to anembodiment of the present invention;

FIG. 5B3 is a flowchart illustrating a handover in anend-to-end/network-controlled mode while the HODF initiates theperforming of the handover according to the structure of FIG. 3B;

FIG. 5C is a flowchart illustrating a handover in a CP-segmented modewhile the HODF initiates the performing of the handover according to anembodiment of the present invention;

FIG. 5D is a flowchart illustrating a handover in a CPandUP-segmentedmode while the HOAF initiates the performing of the handover accordingto an embodiment of the present invention;

FIG. 6A is a flowchart illustrating a procedure of delivering a callfrom a third user to the HODF in the handover UE while an IM Public useridentity (IMPU) of the handover user is shared by the HOSF and the HODFaccording to an embodiment of the present invention;

FIG. 6B is a flowchart illustrating a procedure of delivering a callfrom a third user to the HODF in the handover UE while the IMPU of thehandover user is not shared by the HOSF and the HODF according to anembodiment of the invention;

FIG. 7A is a flowchart illustrating a CS-IMS handover in anend-to-end/terminal-controlled mode according to an embodiment of thepresent invention;

FIG. 7B is a flowchart illustrating a CS-IMS handover in aCP-segmented/terminal-controlled mode when the peer user also is locatedin a CS domain according to an embodiment of the invention;

FIG. 7C is a flowchart illustrating an IMS-CS handover in aCP-segmented/terminal-controlled mode according to an embodiment of thepresent invention;

FIG. 8 is a schematic diagram illustrating a system implementing anIMS-IMS handover when access networks of the HOSF and HODF aredifferent;

FIG. 9 is a schematic diagram illustrating a system implementing anIMS-CS handover in a CP-segmented mode;

FIG. 10 is a schematic diagram illustrating a structure of a UE in whicha Handover Detection Function is configured according to an embodimentof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described in detail hereinafter withreference to embodiments and accompanying drawings.

Inventor of the present invention found that the solution of theexisting UMA standards may have the following disadvantages:

1. The UMA is just a solution on the access network layer withoutprocesses such as IP address allocation, so it is impossible for aterminal to keep a same IP address when it moves. During the movement ofthe terminal, if the IP address allocated to the terminal changes forsome networking factors, it is impossible to avoid new addressregistration and old address de-registration in the application layer,and session interrupt brought by the registration and de-registration,thus it is impossible to guarantee session continuity.

2. The UMA only provides a handover solution on the access networklayer, in other words, it assigns new wireless resources to the terminalafter the handover to maintain the continuity access to the network. Butthe UMA does not give consideration to the bandwidth differences ofdifferent access technologies, and it has no corresponding necessaryprocess in the application layer, such as necessary media attributemodification, Quality of Service (QoS) guarantee, etc. Therefore, inpractical service implementations, it is impossible to guarantee sessioncontinuity.

3. When the user changes his/her access means to the IMS during asession, i.e., changes between “interworking access to the IMS from theCS domain” and “direct access to the IMS through varieties of IP accessnetworks”, since the control signaling between the user and the networkare completely different in these two access means, the UMA on accessnetwork layer is not able to solve this problem.

4. The UNC is a brand new product with brand new technology, thedevelopment of the UNC requires much work, and the terminal also needsto include corresponding protocol support. Still under development, theUNC technology and product are immature and some time away from wideapplication.

As described above, both the IMS and the NGN are networks that supportmultiple access technologies and provide IP multimedia services, and theembodiments of the present invention are applicable to both the IMS andthe NGN. However, to simplify the description, the following paragraphsdescribe the embodiments of the invention in the IMS network only, andthe differences in the NGN are explicated at the end of the descriptionto demonstrate the feasibility of the embodiments of the presentinvention in the NGN.

The embodiments of the present invention are applicable to various caseswhen a handover user accesses to the IMS, including direct accessthrough different IP access networks and interworking access through aCS domain, and provide a handover mechanism to maintain sessioncontinuity. The application of the embodiments of the present inventionis independent of the network of the peer user. In other words, the peeruser can be located in an IMS network or a CS domain/Public SwitchedTelephone Network (PSTN). When the peer user is located in the CSdomain/PSTN, the home IMS network of the handover user selects aninterworking gateway according to the standard IMS-CS interworkingmechanism to implement an interworking call with the peer user.Therefore, to fit in different cases when the peer user is located indifferent networks, in embodiments of the present invention, all thedescriptions are towards the terminal side of the peer user from thepoint of view of the IMS/NGN. The “terminal side of the peer user” hasdifferent meanings when the location of the peer user differs: when thepeer user accesses to the IMS/NGN via different IP access networks, theterminal side of the peer user refers to the peer User Equipment (UE);when the peer user is located in the CS domain/PSTN, the terminal sideof the peer user includes the peer UE, an interworking gateway forinterworking with the network where the peer user are located, and a CSconnection between the peer UE and the interworking gateway.

The session handover of a UE can be controlled by the UE itself.Preferably, the session handover can also be controlled by the networkto guarantee the legality and reliability of the session handover.

FIG. 2A is a schematic diagram illustrating a system when performing asession handover in a terminal-controlled mode according to anembodiment of the present invention. As shown in FIG. 2A, when thesession handover is performed in the terminal-controlled mode, thesystem, which is applicable to a network supporting multiple accesstechnologies, includes at least: an HOSF, an HODF, an HOAF and aHandover Detection Function. The HOSF and the HODF have a correlatedrelationship with each other. The HOAF includes a control plane and auser plane, i.e., an HOAF-CP and an HOAF-UP. The HOSF and the HODF aredirectly connected with the HOAF-CP and the HOAF-UP respectively. Theconnection between the HOSF and the HOAF-CP and the connection betweenthe HOSF and the HOAF-UP respectively correspond to the control planeconnection and the user plane connection established before thehandover. The two connections are illustrated with dotted lines. Theconnection between the HODF and the HOAF-CP and the connection betweenthe HODF and the HOAF-UP respectively correspond to the control planeconnection and the user plane connection established after the handover,and they are illustrated with solid lines.

The connection between the HOSF and the HOAF is referred to as the firstconnection hereinafter, the connection between the HOSF and the HOAF-CPis referred to as the control plane first connection, and the connectionbetween the HOSF and the HOAF-UP is referred to as the user plane firstconnection. The connection between the HODF and the HOAF is referred toas the second connection hereinafter, the connection between the HODFand the HOAF-CP is referred to as the control plane second connection,and the connection between the HODF and the HOAF-UP is referred to asthe user plane second connection.

The HOSF is used for establishing a control plane first connection withthe HOAF-CP and a user plane first connection with the HOAF-UP when thefirst user initiates or accepts a session.

The HODF is used for establishing a control plane second connection withthe HOAF-CP and a user plane second connection with the HOAF-UP when thehandover condition is satisfied.

The HOAF-CP is used for establishing a control plane first connectionwith the HOSF when the first user initiates or accepts a session;establishing a control plane second connection with the HODF when thehandover condition is satisfied; replacing the control plane firstconnection with the HOSF with the control plane second connection withthe HODF, and indicating the HOAF-UP to replace the user plane firstconnection between the HOAF-UP and the HOSF with the user plane secondconnection between the HOAF-UP and the HODF.

The HOAF-UP is used for establishing a user plane first connection withthe HOSF when the first user initiates or accepts a session;establishing a user plane second connection with the HODF when thehandover condition is satisfied; and replacing the user plane firstconnection with the HOSF with the user plane second connection with theHODF according to the indication of the HOAF-CP.

The Handover Detection Function is used for detecting whether thehandover condition is satisfied and triggering a handover when thehandover condition is satisfied. The Handover Detection Function can beimplemented as an independent physical entity, or as a logical entity inthe HOSF or the HODF.

When the Handover Detection Function is implemented as a logical entityin the HODF, the working principle of the system is: the HODF detectswhether the handover condition is satisfied, if the handover conditionis satisfied, the HODF sends a connection establishment request to theHOAF-CP, the HOAF-CP interacts with the HOOF after receiving theconnection establishment request to establish a new session link, i.e.,the second connection.

When the Handover Detection Function is implemented as a logical entityin the HOSF, the working principle of the system is: the HOSF detectswhether the handover condition is satisfied, when the handover conditionis satisfied, the HOSF sends a handover indication to the HODF or theHOAF-CP. If the HOSF sends the handover indication to the HODF, the HODFsends a connection establishment request to the HOAF-CP after receivingthe handover indication; the HOAF-CP interacts with the HODF afterreceiving the connection establishment request to establish a newsession link, i.e., the second connection. If the HOSF sends thehandover indication to the HOAF-CP, the HOAF-CP sends a connectionestablishment request to the HODF after receiving the handoverindication; the HODF interacts with the HOAF-CP after receiving theconnection establishment request to establish a new session link.

After establishing the second connection with the HODF, the HOAF-CP andthe HOSF release the first connection between them, and the two userscan continue the session through the second connection.

Moreover, in order to maintain the media exchange continuity betterduring the handover, the HOAF-UP may also be equipped with media streamduplication and filtering functions, i.e., the HOAF-UP duplicates themedia stream to the handover user and sends the media stream to the HOSFand the HODF simultaneously, filters the media streams from the HOSF andthe HODF, selects a valid part and send the selected valid part to theterminal side of the peer user or present to the peer user.

The functions of the entities in the system shown in FIG. 2A areimplemented in the end-to-end/terminal-controlled mode, which is shownin FIG. 3A, and in the segmented/terminal-controlled mode (including theCP-segmented mode shown in FIG. 3C1 and the CPandUP-segmented mode shownin FIG. 3C2).

FIG. 2B is a schematic diagram illustrating a system when performing asession handover in a network-controlled mode according to an embodimentof the present invention. As shown in FIG. 2B, when the session handoveris performed in the network-controlled mode, the system includes notonly the HOSF, HODF, HOAF-CP, HOAF-UP and the Handover DetectionFunction, which are shown in FIG. 2A, but also an HOCF. The HOSF and theHODF are directly connected with the HOAF-CP and the HOAF-UPrespectively, wherein the connection between the HOSF and the HOAF-CPand the connection between the HOSF and the HOAF-UP respectivelycorrespond to the control plane connection and user plane connectionestablished before the handover, and are illustrated with dotted lines.The connection between the HODF and the HOAF-CP and the connectionbetween the HODF and the HOAF-UP respectively correspond to the controlplane connection and the user plane connection established after thehandover. The two connections are illustrated with solid lines.Meanwhile, the HOCF is respectively connected with the HOSF, the HODFand the HOAF-CP to illustrate the control and interaction relationshipsbetween them.

In the system shown in FIG. 2B, besides the functions shown FIG. 2A, theHOSF, HODF, HOAF-CP, HOAF-UP and the Handover Detection Function mayalso have other functions. The function of the HOSF may also include:sending a handover request to the HOCF and processing an authorizationresult returned from the HOCF. The function of the HODF may alsoinclude: sending a handover request to the HOCF and processing anauthorization result returned from the HOCF.

The HOCF is used for forwarding signal streams between the HOSF and theHOAF, and used for centralized management of handover strategies toperform handover authorization, and further for acquiring relevantinformation by performing signaling interactions with the handover UE inorder to update the handover strategies saved in the HOCF.

The functions of the Handover Detection Function in FIG. 2B are the samewith those shown in FIG. 2A: when the handover condition is satisfied,the Handover Detection Function triggers the handover. When the HandoverDetection Function is a logical entity, it can be set not only in theHOSF or the HODF, but also in the HOCF. When the Handover DetectionFunction is set in the HOCF, the HOCF is able to perform signalinginteractions with the handover UE to update the handover detectioncondition saved in the HOCF.

As shown in the structure illustrated in FIG. 2A, the second connectioncan be initiated by the HODF or the HOAF. Therefore, based on thestructure shown in FIG. 2B, no matter the Handover Detection Function isset in the HOSF, the HODF or the HOCF, as long as a handover indicationis sent to either the HODF or the HOAF-CP when the handover condition issatisfied, and the HODF or the HOAF-CP which receives the handoverindication sends a second connection establishment request to the peerside, after interactions between the HODF and the HOAF-CP, a new sessionlink, i.e., the second connection, can be established.

For example, when the Handover Detection Function is a logical entity inthe HOCF, the working principle of the system is: the HOSF establishes acontrol plane first connection and a user plane first connectionrespectively with the HOAF-CP and the HOAF-UP. When the HOCF detectsthat the handover condition is satisfied, it can send a handoverindication to the HOAF-CP, the HOSF or the HODF. If the handoverindication is sent to the HOAF-CP, the HOAF-CP initiates a connectionestablishment request to the HODF after receiving the handoverindication, and interacts with the HODF to establish a secondconnection. If the handover indication is sent to the HODF, the HODFinitiates a connection establishment request to the HOAF-CP afterreceiving the handover indication, and interacts with the HOAF-CP toestablish a second connection; If the handover indication is sent to theHOSF, the HOSF sends a handover indication message to the HODF afterreceiving the handover indication, the HODF initiates a connectionestablishment request to the HOAF-CP after receiving the handoverindication message, and interacts with the HOAF-CP to establish a secondconnection.

After establishing the second connection with the HODF, the HOAF-CP andthe HOSF release the first connection between them, and the two userscan continue their conversation through the second connection.

The functions of the entities in the system shown in FIG. 2B areimplemented in the end-to-end/network-controlled mode, which is shown inFIG. 3B, and in the segmented/network-controlled mode, which is shown inFIG. 3C.

The HOSF and the HODF in the system may be located in a same terminal ordifferent terminals of the handover user (also referred to as the firstuser herein), and accordingly, they can be different access modules in asame terminal of the handover user, or access modules in differentterminals of the handover user. To simplify the description, the HOSFand the HODF are described hereinafter to be in the handover UE,including two situations above.

As shown in FIG. 10, as the handover UE used in an embodiment of thepresent invention, when the HOSF and the HODF are implemented in a sameterminal of the handover user and the Handover Detection Function is setas a logical entity in the HOSF, the handover UE may include a HandoverProcessing Unit and a Handover Detection Function. The HandoverDetection Function includes a Handover Trigger Condition Detection Unitand a Handover Request Unit.

The Handover Trigger Condition Detection Unit can monitor continuouslyand trigger a handover according to various conditions. For example, itcan continuously check the quality of the received radio signal (e.g.,signal intensity). When the quality degrades to a pre-configuredhandover threshold, it automatically searches whether there is awireless network of the same type as the current radio coverage. Ifthere is not any network of the same type, it continues to searchwhether there are other types of wireless networks covering the area,selects a new radio access network and triggers a handover. Thus a radioaccess network of the same type is a preferable choice, and when thereis no such network, other usable access networks are selected based on apre-configured priority order (usually set in an ascending orderaccording to access charging rate).

The Handover Request Unit is used for initiating a handover request tothe network. For example, after the HODF newly accesses to the networkand obtains a new IP address (the IP address is obtained from differentplaces according to different types of access networks, e.g., in aGeneral Packet Radio Service (GPRS) network, it is obtained from aGateway GPRS Support Node (GGSN)), registers the new IP address on aregister server through the new access network, and initiates a handoverrequest through SIP signaling via the new access network.

The Handover Processing Unit, used for supporting special processing ona signaling plane during the session handover, e.g., interacting withthe HOAF-CP to establish the second connection, or avoiding ringing(i.e., avoiding responding 180), etc.

In addition, the HOAF-CP and the HOAF-UP in the system may havedifferent locations: the HOAF-CP and the HOAF-UP may be both located onthe terminal side of the peer user (also referred to as the second userherein); or the HOAF-CP may be located on the home network side of thehandover user while the HOAF-UP is located on the terminal side of thepeer user; or the HOAF-CP and the HOAF-UP may be both located on thehome network side of the handover user.

When the HOAF-CP and the HOAF-UP are both located on the terminal sideof the peer user, the session between the handover user and the peeruser is born only by the control plane connection between the HOSF orthe HODF in the handover UE and the HOAF-CP and the user planeconnection between the HOSF or the HODF in the handover UE and theHOAF-UP respectively. This mode is hereinafter referred to as anend-to-end mode.

If the HOAF-CP is located on the home network side of the handover userand the HOAF-UP is located on the terminal side of the peer user, on thecontrol plane, the session between the handover user and the peer userneeds to be born by two segments of connections, one is the connectionbetween the HOSF or the HODF and the HOAF-CP, i.e., the control planefirst connection or the control plane second connection, and the otheris the control plane connection between the HOAF-CP and the terminalside of the peer user. As the HOAF-UP is on the terminal side of thepeer user, on the user plane, the session between the handover user andthe peer user is still born by the end-to-end user plane first or secondconnection. This mode is hereinafter referred to as a CP-segmented mode.

If the HOAF-CP and the HOAF-UP are both located on the home network sideof the handover user, besides the control plane and user plane firstconnection and the second connection with the HOSF and the HODF of thehandover user respectively, the HOAF needs to establish another controlplane connection and user plane connection with the terminal side of thepeer user. The session between the handover user and the peer user insuch a situation is born by two segments of connections on both controlplane and user plane. To be specific, one segment is the control planeand user plane first connections or the control plane and user planesecond connections, the other segment is the control plane and userplane connections between the HOAF and the terminal side of the peeruser. This mode is hereinafter referred to as a CPandUP-segmented mode.

The structure of the system will be described in detail hereinafter withreference to the embodiments in the terminal-controlled mode and in thenetwork-controlled mode respectively, wherein the session is born in theend-to-end mode, CP-segmented mode or CPandUP-segmented mode.

Corresponding to FIG. 2A, FIG. 3A is a block diagram logicallyillustrating a system when performing a session handover in anend-to-end/terminal-controlled mode according to an embodiment of thepresent invention. As shown in FIG. 2A and FIG. 3A, when the sessionhandover is performed in the end-to-end/terminal-controlled mode, theHOSF and the HODF are located in the handover UE; the HOAF-CP and theHOAF-UP are located on the terminal side of the peer user. The serviceimplementation mode corresponding to the system logic shown in FIG. 3Ais the end-to-end/terminal-controlled mode.

Corresponding to FIG. 2B, FIG. 3B is a block diagram logicallyillustrating a system when performing a session handover in anend-to-end/network-controlled mode according to an embodiment of thepresent invention. As shown in FIG. 2B and FIG. 3B, the HOSF and theHODF are located in the handover UE, the HOCF is located on the homenetwork side of the handover user, and the HOAF-CP and the HOAF-UP areboth located on the terminal side of the peer user. The serviceimplementation mode corresponding to the system logic shown in FIG. 3Bis the end-to-end/network-controlled mode.

Corresponding to FIG. 2A or 2B, FIG. 3C1 is a block diagram logicallyillustrating a system when performing a session handover in aCP-segmented/terminal-controlled mode or aCP-segmented/network-controlled mode according to an embodiment of thepresent invention. As shown in FIG. 2A, 2B and FIG. 3C1, the HOSF andthe HODF are located in the handover UE, the HOCF and the HOAF-CP arelocated on the home network side of the handover user, and the HOAF-UPis located on the terminal side of the peer user; besides the controlplane first connection and control plane second connection establishedrespectively with the HOSF and the HODF before and after the handover, acontrol plane connection is also established by the HOAF-CP with theterminal side of the peer user. The service implementation modecorresponding to the system logic shown in FIG. 3C1 is the CP-segmentmode; when the HOCF, shown with dotted lines, is included in the system,the system works in the network-controlled mode, otherwise, the systemworks in the terminal-controlled mode.

Corresponding to the system shown in FIGS. 2A and 2B, FIG. 3C2 is ablock diagram logically illustrating a system when performing a sessionhandover in a CPandUP-segmented mode according to an embodiment of thepresent invention. As shown in FIG. 2A, 2B and FIG. 3C2, the HOSF andthe HODF are located in the handover UE, the HOCF, HOAF-CP and HOAF-UPare located on the home network side of the handover user; besides thecontrol plane and user plane first connection and second connectionestablished before and after the handover with the HOSF and the HODFrespectively, a control plane connection and a user plane connection arealso established respectively by the HOAF-CP and the HOAF-UP with theterminal side of the peer user. The service implementation modecorresponding to the system logic as shown in FIG. 3C2 is theCPandUP-segment mode; when the HOCF, shown with dotted lines, isincluded in the system, the system works in the network-controlled mode,otherwise, the system works in the terminal-controlled mode.

With respect to the segmented/terminal-controlled mode or thesegmented/network-controlled mode, as shown in FIG. 3C1 and FIG. 3C2, itmay be concluded from the above logical block diagrams that:

the functions of the HOAF-CP further include: establishing a controlplane connection with the terminal side of the peer user, and performingcorrelative control over different session connections with the handoverUE and the terminal side of the peer user before and after the handoverin a 3^(rd) Party Call Control (3PCC) mode.

Furthermore, with respect to the CPandUP-segmented/terminal-controlledmode or the CPandUP-segmented/network-controlled mode shown in FIG. 3C2,it may be concluded that:

the functions of the HOAF-CP further include: assigning resources toimplement the HOAF-UP in the network, controlling the HOAF-UP toestablish media connections with the handover UE and the terminal sideof the peer user respectively before and after the handover, andcontrolling the HOAF-UP to connect these media connections.

The functions of the HOAF-UP further include: under the control of theHOAF-CP, establishing a media connection with the terminal side of thepeer user, connecting its media connections with the terminal side ofthe peer user and the HOSF respectively before the handover, andconnecting its media connections with the terminal side of the peer userand the HODF respectively after the handover.

In the present embodiment, different implementation plans are providedto satisfy different service requirements through above modes. Among thedifferent modes, the end-to-end/terminal-controlled mode is the simplestone without occupying extra network resources, yet in such a mode thenetwork is not able to control the session handover. The end-to-endmode/network-controlled mode provides basic network control mechanism.Unlike the situation in the end-to-end mode, in the CP-segmented modethe complex handling process of new session establishment and sessionreplacement is screened to the terminal side of the peer user throughthe HOAF-CP set in the network during the session handover, thereforethe terminal side of the peer user only needs to support sessionre-negotiation to implement the media stream redirection required by thesession handover as well as necessary media stream adding/deleting ormedia stream attribute modification. In addition, as the connectionbetween the HOAF-CP and the terminal side of the peer user is unchangedbefore and after the session handover, the service control over thisconnection and the charging on this connection in the IMS in which thesession handover is performed may continue, so that the session handoveris performed more smoothly with respect to the terminal side of the peeruser and the IMS in which the session handover is performed. Based onthe above, when the HOAF-CP is also set in the network in theCPandUP-segmented mode, the terminal side of the peer user is no longerrequired to perform the media stream redirection during the handover.Only when the media capabilities of the HOSF and the HODF are differentdoes the terminal side of the peer user need to support sessionre-negotiation to perform media stream adding/deleting or media streamattribute modification; together with the media stream duplication andfiltering functions which are easier to be implemented by the HOAF-UP onthe network side, the service is performed more smoothly during thehandover.

With respect to the network which performs the handover, the HOSF andthe HODF are represented by different access addresses or access means.The different access addresses correspond to different access points ofthe same access technology. or correspond to different accesstechnologies. The different access means refer to direct access to anetwork that supports multiple access technologies or interworkingaccess through a CS domain. It should be noted that in the presentembodiment a CS call which is converted by the CS/IMS interworkinggateway into an IMS session is regarded as a special means for a UE toaccess the IMS. On the handover user side, the HOSF corresponds to anaccess module of the access point, or the access technology or theaccess means before the handover, and the HODF corresponds to anotheraccess module of the access point, or another access technology oranother access means after the handover. When the terminal of thehandover user remains the same before and after the handover, then theHOSF and the HODF are located in the same handover UE; when theterminals of the handover user are different before and after thehandover, the HOSF is located in the handover UE before the handover,and the HODF is located in the handover UE after the handover.

In the present embodiment, the HOSF and the HODF correspond to a samehandover user, thus the HOSF and the HODF are correlated with eachother, and the correlated relationship is used to identify that the HOSFand the HOSF correspond to a same user.

To explain the correlated relationship between the HOSF and the HODF, itis needed to introduce in the first place the existing IM PrivateIdentity (IMPI), IM PUblic identity (IMPU). IMS subscription, andsubscription identities of a same user in different network subsystems,e.g., the IMPU in an IMS network and the MSISDN in a CS domain, both ofwhich correspond to a same user.

In the IMS, a home network operator assigns one or more IMPIs to a usersubscribing IMS services for registration, authorization, management andcharging. Usually the IMPIs are kept private. In addition, one or moreIMPUs are also assigned to the user subscribing IMS services to identifythe user in the sessions of various services. The IMPUs are announced tothe public. In the IMS network, there are many-to-many mappingrelationships between the IMPIs and IMPUs of user.

On the other hand, to satisfy some special application requirements, anIMS subscription corresponding to multiple IMPIs and IMPUs is alsoallowed in the IMS network. Further more, the subscription identities ofa same user in different network subsystems, i.e., the IMPU of the userin an IMS network and the MSISDN of the user in other networksub-systems such as a CS domain, are managed and assigned by the homeoperator of the user, they can be associated with each other.

Thus, the HOSF and the HODF in the present embodiment may be correlatedwith or correspond to each other through various means, e.g., the HOSFand the HODF may respectively correspond to different public identitiesunder a same private identity of an IMS user; or the HOSF and the HODFmay respectively correspond to different private identities under a samepublic identity of the IMS user; or the HOSF and the HODF may correspondto different private identity and public identity respectively, but theprivate identity and the public identity are under the same subscriptionof a user in the IMS network; or the HOSF and the HOSF may be associatedwith each other through the subscriptions of a same user in differentsubsystems on the operator layer, for example, the HOSF and the HODF mayrespectively correspond to the public identity/private identity of auser in IMS network and the MSISDN/International Mobile SubscriberIdentity (IMSI) of the user in CS domain, etc.

As described above, a voice call of a user may be implemented through aVoice over IP (VoIP) born by IP under the control of the IMS, or througha traditional CS domain call, while the access technology through whichthe UE accesses the IMS may not have continuous coverage like the WCDMAsystem (e.g., the WLAN adopts hot spot coverage), so when a useraccesses the IMS through the WLAN to make a VoIP call and roams out ofthe hot spot coverage of the WLAN, the network connection will be lost.However, the WCDMA system, which is designed to cover wide areascontinuously, has its own disadvantages in some buildings due topenetration loss of radio signals, while hot spot coverage technology,such as the WLAN, is able to fill the coverage holes emerged here.Therefore, the session handover between the IMS and CS domain shouldalso be taken into consideration. In the present embodiment, a CS callwhich is converted by the CS/IMS interworking gateway into an IMSsession is regarded as a special means for a UE to access the IMS.Therefore, the present embodiment covers not only the handover from anIMS to another IMS, but also the handover between an IMS and a CSdomain.

Firstly, the service interworking between the IMS and the CS domain willbe explained.

The core network of a Universal Mobile Telecommunications System (UMTS)is divided into a CS domain and a PS domain, which performs CS services(such as voice, circuit data and fax services) and PS servicesrespectively. In UMTS Release 5, an IMS domain is introduced, signalingand media streams of the IMS domain are born by the PS domain. Accordingto the IMS architecture definitions, interworking with the CS network,i.e., the PSTN or the PLMN CS is performed through an interworkinggateway Media Gateway Control Function/IP Multimedia-Media Gateway(MGCF/IM-MGW). The MGCF is used for performing a control planeinteraction between the CS domain and the IMS, supporting protocolinteraction and call interworking between ISUP/Bearer Independent CallControl (BICC) and the SIP, controlling the IM-MGW to performinterworking between a Time Division Multiplex (TDM) hearer of the CSnetwork and a user plane broadband/narrow band IP bearer of the IMSdomain through H.248, and accomplishing necessary Codec transformations.

FIG. 4 is a schematic diagram illustrating connection relationships whenan IMS session is transferred to a CS call. When a VoIP call controlledby the IMS is transferred to a CS call, the actual connections are shownin FIG. 4. At this time, from the point or view of the IMS network, theCS call which is converted by the CS/IMS gateway into an IMS session maybe regarded as a special means for a UE to access the IMS, therefore theIMS-CS handover is equivalent to the IMS-IMS handover between the IMSsession 1 and IMS session 2 which points to the CS/IMS interworkinggateway shown in FIG. 4.

It can be concluded from the above that in practical implementations, ifboth the HOSF and the HODF on the handover user side access the IMSnetwork directly through varieties of IP access networks to establish asession with the terminal side of the peer user, it is an IMS-IMSsession handover that is performed in the present embodiment; if theHOSF establishes a session with the terminal side of the peer userthrough a CS/IMS interworking gateway between the CS domain and the IMSnetwork, while the HOOF accesses the IMS network directly throughvarieties of IP access networks to establish a session with the terminalside of the peer user, it is a CS-IMS session handover that is performedin the present embodiment; and if the HOSF accesses the IMS networkdirectly through varieties of IP access networks to establish a sessionwith the terminal side of the peer user, while the HODF establishes asession with the terminal side of the peer user through a CS/IMSinterworking gateway between the CS domain and the IMS network, it is anIMS-CS session handover that is performed in the present embodiment.

Several embodiments of different service modes in different sessionhandover circumstances will be described hereinafter.

In the flowcharts of the following embodiments, UE A and UE A′ areterminals of the handover user, in other words, the terminals of thehandover user before and after the handover respectively. The UE Acorresponds to the HOSF and the UE A′ corresponds to the HODF. UE B isthe terminal of the peer user, in other words, the second UE. UE C isthe terminal of a third user; IMS Network (A) includes the home IMSnetwork of the handover user and the IMS network where the handover useris currently located excepting the case that the handover user accessesthe IMS through CS interworking; IMS Network (B) includes the home IMSnetwork of the peer user and the IMS network where the peer user iscurrently located; CS Network (A) is the home CS network of the handoveruser, or the CS network where the handover user is currently located; CSNetwork (B) is the home CS network of the peer user, or the CS networkwhere the peer user is currently located.

The IMS-IMS session handover is described hereinafter with reference toan embodiment of the present invention.

FIG. 5A is a flowchart illustrating when both the handover UE and thepeer UE are located in the IMS, a handover is performed in anend-to-end/terminal-controlled mode, while the HODF initiates theperforming of the handover, i.e., initiates the establishment of the newconnection between the HODF and the HOAF-CP, according to an embodimentof the present invention.

This embodiment is an IMS-IMS session handover in theend-to-end/terminal-controlled mode, wherein the HOSF detects whetherthe handover condition is satisfied and indicates the HODF to performthe handover.

As shown in FIG. 3A and FIG. 5A, the IMS-IMS handover in theend-to-end/terminal-control led mode includes the following steps:

Step 5101: during the establishment of an initial session, the HOSF inthe handover UE establishes an initial session connection with the HOAFon the terminal side of the peer user via the IMS.

The initial session connection established by the HOSF and the HOAFincludes a control plane connection established between the HOSF and theHOAF-CP, and a user plane connection established between the HOSF andthe HOAF-UP; the IMS network includes the IMS network that the handoveruser involves in, and the IMS network that the peer user involves in,i.e., the home IMS networks of the two users and the IMS networks wherethe two users are currently located. Since the peer user is also in anIMS network, the terminal side of the peer user herein is referred asthe peer UE.

Step 5102: during the conversation between the handover UE and the peeruser through the HOSF, the HODF in the handover UE successfullyregisters in the IMS network (e.g., as the handover user roams, the HODFfinds another IP access network and successfully accesses the IMS).

Based on the correlated relationship between the HOSF and the HODFsubscribed in the IMS, a same service node is guaranteed to be assignedto the HODF when the HODF accesses the IMS network according to astandard service node distribution way during registration of an IMSnetwork user.

Step 5103: the HOSF detects a handover-related event.

The step of the HOSF detecting a handover-related event includes: theHOSF detects that it will lose the connection to the IMS network, e.g.,the radio signal intensity degrades to a pre-configured threshold.Optionally the battery in the local end is about to be used up when theHODF is located in another handover UE. Optionally the HOSF detects thatthe HODF has successfully accessed the IMS network, e.g., the HOSFlearns through interactions of internal modules in the same terminal orthrough a user indication that the HODF has successfully accessed thenetwork, and the radio signal quality, or the network priority level orthe service charging rate of the network that the HODF accesses arebetter than those of the network that the HOSF accesses, or the HOSFdetects that the handover condition pre-configured by the user issatisfied, e.g., the conversation duration reaches a pre-configuredhandover interval, or the HOSF detects a handover indication from theuser, or any combination of the above;

Step 5104: the HOSF sends a handover indication to the HODF.

The handover indication includes a location identifier of the HOAF, andinitial session information between the HOSF and the HOAF;

Step 5105: the HODF acquires the location identifier of the HOAF and theinitial session information from the handover indication, and returns ahandover indication confirmation to the HOSF;

As described above, the HOSF and the HODF can be located in differentmodules of a same UE or in different UEs, when they are located indifferent UEs, the interaction between them is performed through the SIPor a Service Location Protocol (SLP) via short distance wirelesscommunication technologies, such as Bluetooth; when they are located inthe same UE, they can interact with each other through various internalinteraction methods, e.g., interacting through internal messages amonginternal modules or through visiting a shared internal memory, etc.

When the HOSF and the HODF are located in different modules in the sameUE, the interaction between them is the interaction among internalmodules within a physical entity, the present embodiment is only apreferred embodiment illustrating an interaction requirement; the twomodules can interact through any convenient internal interaction method,as long as the method provides equivalent effect and is in accordancewith the essence of the present embodiment.

The relationship and interaction between the HOSF and the HODF in thefollowing embodiments are identical to the description above. However,in order to simplify the description, the HOSF and the HODF are supposedto be in different modules of the same UE.

Step 5106: the HODF constructs a session establishment request, whichcarries initial session information and handover indication information,and sends the session establishment request to the HOAF through the IMSnetwork.

The session establishment request includes the location identifier ofthe HOAF. Network entities in the IMS network decide routing accordingto the location identifier of the HOAF, and accomplish some specialprocesses according to the handover indication information, includingprohibiting the duplicate triggering of service, and/or avoidinginfluences from other services, and/or special charging processes, etc.

In the session establishment request, an indication declaring that thenew session is used to replace the original session between the HOSF andthe HOAF, and session related information may be carried through theREPLACES header field.

Step 5107: the HOAF judges whether the session establishment request islegal; if it is legal, execute Step 5108; otherwise, the HOAF returns afailure response to the HODF to end the procedure.

Since the terminal side of the peer user cannot obtain the subscriptiondata of the handover user in the end-to-end/terminal-control mode, it ishard to perform checking on correlated relationship between the HOSF andthe HODF, i.e., it is impossible to determine whether the HOSF and theHODF belong to the same user. Therefore, the step of the HOAF judgingwhether the session establishment request is legal is performedaccording to the initial session information carried in the sessionestablishment request.

The step of the HOAF judging whether the session establishment requestis legal includes: the HOAF judges whether the original session to bereplaced and indicated by the initial session information corresponds tothe initial session established between the HOAF and the HOSF in thehandover UE according to the session information carried by the sessionestablishment request, so as to determine whether the sessionestablishment request used for the handover is legal.

In addition, after receiving the session establishment request, the HOAFcan further instruct the peer UE to avoid ringing according to thehandover indication information.

Step 5108: the HOAF interacts with the HODF for the information neededin the establishment of the new session, and replaces the initialsession with the new session.

The step of replacing the initial session with the new session includes:replace the media stream exchanged with the HOSF in the initial sessionwith the media stream exchanged with the HODF; add/delete media streamsand modify the attribute of the media stream, such as media type orcoding format, etc., required by the change of the media capabilitiesbetween the HODF and the HOSF through session media re-negotiation.

Step 5109: the HOAF returns a session answer response to the HODF.

Step 5110: the HODF returns a session answer response acknowledgment tothe HOAF.

Now the new session connection between the HODF and the HOAF isestablished, and the initial session connection with the HOSF isreplaced by the new session connection on the HOAF side.

Step 5111: the HOAF sends a session release message to the HOSF.

Step 5112: the HOSF returns a session release success message to theHOAF, the HOSF and the HOAF release the session connection between them.

Now the handover user and the peer user continue their conversationthrough the new connection between the HODF and the HOAF.

It should be noted that, although in the present embodiment the peer UEis located in the IMS network, actually the method described in theabove embodiment is also applicable to the cases when the peer UE islocated in a PLMN CS domain or in the PSTN, so far as the terminal sideof the peer user is taken as the combination of the peer UE, the CS/IMSinterworking gateway which performs interworking for the peer UE and theCS connection between them. At this time, the call to the terminal sideof the peer user is processed according to the standard IMS-CSinterworking process, in other words, is directly routed by the home IMSnetwork of the handover user to the network where the peer UE is locatedand further to the peer UE.

That is to say the schemes are independent of the network that the peerUE accesses, therefore in the embodiments described hereinafter, allpeer users are supposed to be located in the IMS network, unless thereis a special reason that needs to be explained.

FIG. 5B1 is a flowchart illustrating a handover according to thestructure of the FIG. 3B.

The handover when there is an HOCF is described in detail hereinafterwith reference to FIG. 5B1 and an embodiment.

As shown in FIG. 5B1, in the present embodiment, a normal session, i.e.,the first connection, has already been established between the HOSFwhich has accessed the network and the peer UE. The session handover inthe end-to-end/network-controlled mode according to the embodimentincludes the following steps:

Step 5B101: the handover UE reports a service event to the HOCF.

The handover UE includes the HOSF and the HODF, and there are multiplemethods for the handover UE to report the service event to the HOCF onthe application layer, e.g., a Presence mechanism or directly access adatabase, etc.

The service event includes one of the following cases or any combinationof the following:

(1) when the session connection of the handover UE changes, reportsession link change information, e.g., when the HOSF successfullyestablishes the initial session connection with the peer side, the HOSFreports the address of the peer side and the status of the establishedconnection to the HOCF, the HOCF saves and maintains address of the peerside and the status of the established connection of the HOSF associatedwith each user, and sends the address of the peer side and the status ofthe established connection to the HODF during the handover to establishthe new session link;

(2) when the HODF enters into the coverage area of a new access network,and if the handover successfully accesses the new access network andobtains a new access point, the HODF reports the address of the newaccess point and this event to the HOCF;

(3) When the HOSF detects that the signal intensity of the originalnetwork degrades to a threshold, the HOSF reports this situation to theHOCF, in other words, it gives a communication quality falling alert,and in such a case a handover may be performed.

(4) When the handover user triggers a handover on his own initiative, ahandover request is reported by the handover UE to the HOCF; this mayhappen when the handover user needs to transfer to other mode or adoptservices provided by other operators, at this time, the HOCF needs toverify the handover condition;

(5) the handover UE may also trigger a handover in follow circumstances:the handover UE moves, or needs to transfer to a new access network dueto pre-configured conditions of the user or the network operator. Atthis time, the handover UE triggers a handover request to the HOCF.There may be many pre-configured conditions, e.g., when the handover UEdetects that the signal of the original access network deteriorates, itactively triggers a “signal weak handover request”, and this kind ofrequest is basically handled completely by the radio access networkaccording to the prior art; or, when the handover UE enters into thecoverage area of a new access network, it triggers an access request andmakes a connection request attempt, including obtaining address andaccess verification, etc., if the connection request attempt succeeds,it triggers a “handover possibility event” to the HOCF for processing.

Actually, the handover may be performed in various cases including notonly the signal deterioration of a cell caused by roaming, but alsoconditions pre-configured by the user, e.g., when there are multipleaccess networks, the user may assign higher priority to a network whichprovides plenty of services, at this time, a handover is needed. In aword, the handover strategies can be more accurate and flexible, forinstance, a handover can be triggered according to signal intensity,operator priority, access network priority, time and location, etc. Forexample, for a dual-mode cell phone that supports a Global System ofMobile (GSM) and a Wideband Code Division Multiple Access (WCDMA)system, when there is a coverage of the GSM and a coverage of the WCDMAsimultaneously and the GSM is the current serving-network, the userusually wants to switch to the WCDMA network to obtain richer serviceswith higher quality.

In this embodiment, the system may further perform a handover conditionupdate, i.e., the handover user updates the handover conditions and userhandover strategies by interacting with the HOCF through the handoverUE. The HOCF maintains the handover conditions and strategies andinitiates a handover when receiving a notification of a related event,or performs an authorization when receiving a handover request.

Step 5B102: after receiving the service event from the handover UE, theHOCF determines whether to initiate a handover according to thepre-configured handover conditions; the handover conditions can besignal intensity, operator preference, charging strategy or userpre-configured strategies, or any combination of the above. The HOCFthen informs the handover UE by returning a handover initialization or ahandover refuse command; if a handover refuse command is returned, go toStep 5B103, otherwise, go to Step 5B104.

Step 5B103: if the handover condition is not satisfied, the HOCF needsto inform the handover UE in some cases, e.g., when the handover useractively triggers a handover request, the HOCF should return a handoverrefuse message to the handover user, and ends the procedure.

Step 5B104: the HOCF indicates the HODF to perform the handover andestablish a new session link; the HODF also receives other informationfrom the HOCF at the same time along with the handover indication,wherein the most important information is the address of the peer UE,which is reported in Step 5B101; the HODF establishes a new sessionlink, i.e., the second connection, on its own with the peer side of theoriginal session link, and the process of the establishment can beimplemented without the intervention of the service control layer.

Step 5B105: after the establishment of the second connection, the HODFreports the result to the HOCF; if the new session link is successfullyestablished, go to Step 5B106, otherwise, go to Step 5B107.

Step 5B106: the HOCF informs the HOSF to release the original sessionlink, i.e., the first connection; the HOSF release the original sessionlink with the peer side on its own, thus the service is transferred tothe new session link, end the procedure. The step of releasing theoriginal link can be implemented without the intervention of the servicecontrol layer.

Step 5B107: after receiving new session link establishment failureinformation from the HODF, the HOCF releases the resources used forsession link establishment, informs the handover user of the new sessionlink establishment failure information, and end the procedure.

When the handover UE is controlled by a service control point, thehandover is described hereinafter with reference to FIG. 5B2.

As shown in FIG. 5B2, the HOSF establishes a normal session link withthe second UE on the other side through an access network in thisembodiment.

Step 5B201: when the handover UE needs to be switched to a new accessnetwork due to movement or pre-configured conditions of the user or theoperator, trigger a handover request to the service control point.

As the embodiment described above, there can be many handoverconditions, which will not be described herein.

Step 5B202: after receiving a signal weak handover request event or ahandover possibility event or other events from the access network, theservice control point triggers the event to the corresponding HOCF. Inother words, the service control point reports the handover request tothe HOCF, which is responsible for centralized handling of handoverrequests in the entire network.

Step 5B203: after receiving the trigger event from the service controlpoint, the HOCF determines whether to initiate a handover according tothe pre-configured handover conditions; the handover condition may besignal intensity, operator preference, charging strategy or userpre-configured strategies, or any combination of the above. The HOCFthen returns a handover indication or handover refuse command to theservice control point; if a handover refuse command is returned, go toStep 5B204, otherwise, go to Step 5B205.

Step 5B204: after receiving the handover refuse command, the servicecontrol point issues a “Handover Refuse Command” to the handover UE atthe original access point. The handover UE at the original access pointends the current handover request procedure, and returns a processingresult to the service control point. For instance, the handover UE canmodify the pre-configured handover condition due to the handoverfailure, and return the modification event to the service control point.End the current handover procedure.

Step 5B205: after receiving the handover indication, the service controlpoint controls the handover UE to establish a new session link with theother end of the session. The step of establishing the new session linkincludes: issue a command to the handover UE, require the handover UE tocall the other end of the session actively following a standardprocedure, and establish a new session link, which uses the address andother relevant resources of the handover UE in the new access network,through the service control point of the other end of the session.Optionally, the service control point controls other network componentswhich are able to provide conference resources to establish aconference, and establishes an additional session link for the handoverUE and the other end of the session through a standard procedure, andcontrols them to join the conference.

Step 5B206: after receiving the control command from the service controlpoint, the handover UE at the new access point calls to establish thenew session or joins in the conference cooperating with the command, andreturns the processing result to the service control point.

Step 5B207: the service control point performs corresponding processaccording to the processing result about the establishment of the newsession link returned by the handover UE, if the processing result isthat the link is successfully established, sends a command to thehandover UE at the original access point to release the first session,go to Step 5B208, otherwise, go to Step 5B209.

Step 5B208: after receiving the command of releasing the originalsession link, the handover UE at the original access point releases theresources occupied by the original session link, transfers the serviceborn by the original session to the new session for subsequent process,and returns a releasing result to the service control point to informthe HOCF that the handover is accomplished; the HOCF continues with thesubsequent process, such as recording current access mode and status.

Step 5B209: the service control point reports handover failureinformation to the HOCF, and completes the release of the relatedresources employed in the handover, returns a handover failureinformation to the handover UE at the same time, maintains the originalsession link, releases all the resources employed for the establishmentof the new session link, ends the procedure.

The above two embodiments mainly show the handover initiated by the HOCFaccording to the detection of the information reported by the handoverUE, and also show different establishment methods of the new end-to-endsession link during the handover. The embodiment hereinafter shows anIMS-IMS handover in the end-to-end/network-controlled mode, wherein theHOSF detects that the handover condition is satisfied and sends ahandover request to the HOCF, which authorizes the request and returns aconfirmation to the HOSF. Then the HOSE instructs the HODF to performthe handover.

FIG. 5B3 is a flowchart illustrating a handover in anend-to-end/network-controlled mode while the HODF initiates theperforming of the handover according to the structure of FIG. 3B. TheIMS-IMS handover in the end-to-end/network-controlled mode will bedescribed hereinafter with reference to accompanying FIG. 5B3 and FIG.3B, including:

Step 5201: the HOSF establishes a session connection with the HOAF onthe terminal side of the peer user during the establishment of theinitial session, and the HOCF is triggered in the home IMS network ofthe handover user according to the evaluation of the iFC in the usersubscription data;

During the establishment of the initial session, the service nodeS-CSCF, which is currently assigned to the handover user by his/her homeIMS network, triggers the current session to a network entity thatperforms the functions of the HOCF, such as an AS, according to the iFCin the subscription data downloaded from the HSS during the userregistration according to the existing service triggering method in theIMS.

Step 5202: the HODF in the handover UE accesses the IMS network duringthe session, and the HOSF learns through interactions among internalmodules in the handover UE or through a user indication that the HODFhas already accessed the network.

Step 5203: the HOSF detects a handover-related event, e.g., the signalof the local point is weaker than that of the network which the HODFaccesses.

Step 5204: the HOSF sends a handover request to the HOCF.

The handover request carries session information, handover reasoninformation and HODF identity information.

Step 5205: the HOCF performs a handover authorization, i.e., determineswhether the handover request is legal; if the handover request is legal,the HOCF returns a handover confirmation to the HOSF, execute Step 5206,otherwise, the HOCF returns a handover refuse to end the procedure.

The handover authorization performed by the HOCF can be a step ofdetermining whether the HOSF is correlated with the HODF, the detailedprocess includes: the HOCF determines, according to the informationcarried in the handover request, e.g., the respective IMPU of the HOSFand the HODF, whether the two IMPUs are different public identities of asame IMS user's private identity; if they are, it is indicated that theHOSF is correlated with the HODF, otherwise, the HOSF is not correlatedwith the HODF.

The steps from Step 5206 to Step 5214 are basically identical to thesteps from Step 5104 to Step 5112, the only difference lies in Step5104, wherein the HOSF directly sends a handover indication to the HODFafter the detection of the handover-related event, while in Step 5206the HOSF sends the handover indication to the HODF after receiving aconfirmation message from the HOCF when the authorization passes.

The handover in segmented mode is described hereinafter with referenceto two embodiments of the present invention.

This embodiment is an IMS-IMS session handover in theCP-segmented/terminal-controlled mode, wherein the HODF determineswhether the handover condition is satisfied and directly performs thehandover.

FIG. 5C is a flowchart illustrating a handover in a CP-segmented modewhile the HODF initiates the performing of the handover according to anembodiment of the present invention. As shown in FIG. 5C, the IMS-IMShandover in the CP-segmented/terminal-controlled mode includes:

Step 5301: during the establishment of the initial session, the HOSFsends a session establishment request to the home IMS network of thehandover user, the IMS network triggers the control function of an ASthat performs the functions of the HOAF-CP according to the evaluationof iFC in the user subscription data, the HOAF-CP ends the sessionestablishment request from the HOSF, interacts with the HOSF toestablish a first session segment, SEG1, initiates and establishes asecond session segment, SEG2, with the terminal side of the peer user.

During the establishment of the initial session, the service nodeS-CSCF, which is currently assigned to the handover user by his/her homeIMS network, triggers the current session to the AS that performs thefunctions of the HOAF-CP according to the iFC in the subscription datadownloaded from the HSS during the user registration following theexisting service triggering method in the IMS. In order to reduce theinfluence to other IMS services, the HOAF-CP should be set as close aspossible to the HOSF/HODF, i.e., when the HOSF is a calling party duringthe establishment of the initial session, the priority level of the iFCcorresponding to the AS that performs the functions of the HOAF-CPshould be set as high as possible; when the HOSF is a called partyduring the establishment of the initial session, the priority level ofthe iFC corresponding to the AS that performs the functions of theHOAF-CP should be set as low as possible.

The HOAF-CP controls the establishment of the segmented session betweenthe HOSF and the terminal side of the peer user, and performscorrelative control over the session segments SEG1 and SEG2 in the 3PCCmode, so as to implement the end-to-end media exchange between the HOSFand the HOAF-UP on the terminal side of the peer user.

Step 5302: during the session between the handover user and the peeruser through the HOSF, the HODF accomplishes an IMS access, and the HOSFlearns that through interactions among internal modules in the handoverUE or a user indication;

Step 5303: the HOSF sends a handover event notification to the HODF whenthe HOSF detects a handover event.

The handover event may include: the HOSF is about to lose the connectionto the IMS network, and/or user pre-configured handover strategies,e.g., the conversation duration reaches a pre-configured handoverinterval. In addition, when the HOSF sends the handover eventnotification to the HODF, relevant information of the initial sessionbetween the HOSF and the HOAF is carried in the handover eventnotification;

Step 5304: the HODF determines whether the handover condition issatisfied, if the condition is satisfied, the HODF returns an eventnotification confirmation to the HOSF, otherwise, do not perform thehandover and end the procedure.

The handover condition is considered to be satisfied under the followingsituations: the HODF considers that it has successfully accessed the IMSnetwork, and/or the HODF determines that it can process a session,and/or the access point of the HODF is better than that of the HOSF withrespect to signal intensity, network priority or service fee.

Step 5305: the HODF sends a session establishment request, which carriesthe session information and the handover indication, to the HOAF-CPthrough the IMS network.

The HODF sends the session establishment request to the HOAF-CPaccording to the session information in the handover event notification,and the session establishment request carries the location identifier ofthe network entity that performs the functions of the HOAF-CP; thelocation identifier can be provided by the HOSF to the HODF in thehandover event notification, or be pre-stored as a part of networkservice settings, or be issued by the network when the HODF registers inthe network and then stored in the HODF; In the IMS network, the sessionestablishment request will be routed to the HOAF-CP by relevant entitiesaccording to the location identifier.

Step 5306: the HOAF-CP judges whether the session establishment requestis legal; if it is legal, execute Step 5307; otherwise, the HOAF-CPreturns a failure response to the HODF and ends the procedure.

Step 5307: based on a media negotiation situation during theestablishment of the session between the HOAF-CP and the HODF, theHOAF-CP sends a re-negotiation request to the terminal side of the peeruser through the SEG2 between itself and the terminal side of the peeruser.

Step 5308: the terminal side of the peer user performs a re-negotiationinteraction with the HOAF-CP, and returns a re-negotiation response tothe HOAF-CP, which in turn returns a session answer response to theHODF.

Step 5309: the HODF returns a session answer response acknowledgement tothe HOAP-CP, which in turn returns a re-negotiation responseacknowledgement to the terminal side of the peer user.

From Step 5305 to Step 5309, the HOAF-CP correlatively controls theestablishment of the SEG3 between itself and the HODF and there-negotiation on the SEG2 bet ween itself and the terminal side of thepeer user in the 3PCC mode, so as to make the HOAF-UP on the terminalside of the peer user change to exchange with the HODF finally, and thetype of the exchanged media is compatible to the media capability of theHODF.

Thus, the new session connection is established between the HOAF and theHODF, and the initial connection with the HOSF is thus replaced by thenew session connection; in the following steps the initial session willbe released.

Step 5310: the HOAF sends a session release message to the HOSF throughthe SEG1.

Step 5311: the HOSF returns a session release success message to theHOAF through the SEG1, the HOSF and the HOAF release the sessionconnection between them.

It should be noted that the CP-segmented mode in this embodiment canalso work in the network-controlled mode, wherein, the home IMS networkof the handover user triggers not only the control of the HOAF-CP, butalso the control of the HOCF for the handover authorization, and the twocontrol functions usually are performed in the same AS. During thehandover event detection and handover initiation, when the entity whichis in charge of condition detection (e.g., the HODF in this embodiment)detects that the handover condition is satisfied, it needs to send ahandover request to the HOCF first, and performs handover only after therequest authorization on the HOCF passes.

The next embodiment shows an IMS-IMS handover in aCPandUP-segmented/network-controlled mode, wherein the HOSF detects thatthe handover condition is satisfied and sends a handover request to theHOCF, the HOCF indicates the HOAF to perform the handover after theauthorization succeeds.

FIG. 5D is a flowchart illustrating a handover initiated in aCPandUP-segmented mode while the HOAF initiates the performing of thehandover according to an embodiment of the present invention. As shownin FIG. 5D, the IMS-IMS handover in theCPandUP-segmented/network-controlled mode according to the embodiment ofthe present invention includes the following steps:

Step 5401: the HOST sends a session establishment request to the homeIMS network of the handover user during the establishment of the initialsession.

Step 5402: the IMS network triggers, according to the iFC in the usersubscription data, the control function of the AS that performs thefunctions of the HOCF and the HOAF-CP, the HOAF-CP controls theestablishment of the segmented session between the HOSF and the terminalside of the peer user, including the SEG1 between the HOAF-CP and theHOSF, and the SEG2 between the HOAF-CP and the terminal side of the peeruser.

Step 5403: during the establishment of the segmented session, theHOAF-CP assigns corresponding media resources on the network side as theHOAF-UP for the session, and establishes segmented media exchangebetween the HOSF and the terminal side of the peer user through theHOAF-UP over the two session segments separated by the HOAF-CP.

The step of the HOAF-CP assigning media resources as the HOAF-UP for thesession includes: assign a corresponding port on an IMS Media ResourceFunction (MRF), and accomplish the segmented media exchange between theHOSF and the terminal side of the peer user, wherein, the segmentsinclude: from the HOSF to an HOAF-UP port 1, from the HOAF-UP port 1 toan HOAF-UP port 2 and from the HOAF-UP port 2 to the terminal side ofthe peer user. Among the segments, the media exchange between the HOSFand the HOAF-UP port 1 is controlled by the SEG1 between the HOSF andthe HOAF-CP, the media interaction between the HOAF-UP port 2 and theterminal side of the peer user is controlled by the SEG2 between theHOAF-CP and the terminal side of the peer user, and the connectionbetween the HOAF-UP port 1 and HOAF-UP port 2 is controlled by theHOAF-CP.

Thus, the initial session connection in the CPandUP-segmented mode isestablished between the HOSF and the terminal side of the peer user totransmit the session. During the session between the handover user andthe peer user through the HOSF, the HODF accesses the network, and theHOSF learns that through interactions among the internal modules in thesame terminal of a handover user or a user indication.

Step 5404: the HOSF detects that the handover condition is satisfied,and then sends a handover request to the HOCF.

The handover request carries session information, handover reasoninformation and HODF information.

Step 5405: the HOCF performs a handover authorization to determinewhether the handover is legal; if it is legal, execute Step 5406,otherwise, the HOCF returns a handover refuse message to the HOSF andends the procedure.

The step of HOCF performing the handover authorization to determinewhether the handover is legal can be implemented by determining whetherthe HOSF is associated with the HODF, i.e., determining, according tothe information, e.g., the IMPI, of the HOSF and the HODF carried in thehandover request and in the user subscription data (saved in the HOCF,or saved in the HSS and downloaded to the HOCF when the user registers),whether the IMPIs of the HOSF and the HODF are corresponding IMPIs of asame IMPU; if they are, it indicates that the HOSF is associated withthe HODF, otherwise, it indicates that the HOSF is not associated withthe HODF.

Step 5406: the HOCF indicates the HOAF-CP to perform the sessionhandover.

When the HOCF indicates the HOAF-CP to perform the session handover, itprovides the location identifier of the HODF, obtained from the handoverrequest or the user subscription data, to the HOAF-CP.

Since the HOCF and the HOAF-CP are located in the same network entity(implemented in the same AS), the above interaction between the HOCF andthe HOAF-CP is also interactions among internal modules, and theforegoing is only a preferred embodiment illustrating an interactionrequirement. The interactions between the HOCF and the HOAF-CP can beimplemented through any convenient internal interaction process, as longas the process achieves the equivalent effect and is in accordance withthe essence of the embodiment of the present invention.

Step 5407: the HOAF-CP sends a session establishment request to the HODFthrough each IMS network entity according to the obtained locationidentifier of the HODF.

The session establishment request carries the location identifier of theHODF, the HOAF-CP and each IMS network entity decide the route accordingto the location identifier of the HODF. Furthermore, the sessionestablishment request can carry a handover indication, and each IMSnetwork entity performs special processes on the session according tothe handover indication, including: prohibiting the duplicate triggeringof service, and/or shunning influences from other services, and/orspecial charging, etc.

Step 5408: the session establishment request can further carry relevantinformation of the HOSF, and the HODF determines whether the sessionestablishment request is legal according to the relevant information ofthe HOSF in the session establishment request; if the request is legal,execute Step 5409, otherwise, the HODF returns a refuse response to theHOAF-CP and ends the procedure.

The step of the HODF determining whether the session establishmentrequest is legal includes: the HODF determines whether the HOSF is theassociated HOSF of the HODF. In addition, each IMS network entity canalso perform a special process on the session according to the relevantinformation of the HOSF in the session establishment request (regard therelevant information as a kind of implicit handover indication),including: prohibiting the duplicate triggering of service, and/orshunting influences from other services, and/or special chargingtreatment, etc.

Step 5409: the HODF interacts with the HOAF-CP to establish a newsession SEG3 between them, and during the establishment, the HODFperforms a media negotiation with the HOAF-CP according to the mediacapability of the local end to determine the media type, coding format,address and port or other information of the exchanged media. During theestablishment of the SEG3, the HOAF-CP assigns a new port resource,i.e., HOAF-UP port 3, on the MRF, and controls the establishment of themedia exchange between the HODF and the HOAF-UP port 3.

The HODF can also perform a special process according to the handoverindication (including the relevant information of the HOSF as animplicit handover indication) in the session establishment request,e.g., controls the handover UE to avoid ringing.

Step 5410: the HODF returns a session answer response to the HOAF-CP.The HOAF-CP determines whether to modify the attribute of the mediaexchanged with the terminal side of the peer side according to theresult of the media negotiation with the HODF. In the presentembodiment, the HOAF-CP initiates and interacts to accomplish there-negotiation through the SEG2 with the terminal side of the peer user.

During the re-negotiation through the SEG2, the HOAF-CP modifies theattribute of the media exchanged between the HOAF-UP port 2 and theterminal side of the peer side according to the result of the medianegotiation with the HODF. Finally, under the control of the HOAF-CP,replace the connection between the HOAF-UP port 1 and the HOAF-UP port 2by the connection between the HOAF-UP port 2 and the HOAF-UP port 3, andrelease the HOAF-UP port 1 during the release of the SEG1.

Step 5411: the HOAF-CP returns a session answer response acknowledgementto the HODF, and returns a re-negotiation answer responseacknowledgement to the terminal side of the peer user.

Thus, the session between the HODF and the HOAF is established, and theinitial session between the HOSF and the HOAF is replaced.

Step 5412: the HOAF-CP sends a session release message through the SEBGto the HOSF, the HOSF then releases the connection of the SEG1 with theHOAF-CP, and returns a session release success message to the HOAF-CP.

Now the handover user and the peer user continue their conversationthrough the new connection between the HODF and the HOAF.

It can be seen from the above embodiment that in the establishment ofthe new connection between the HODF and the HOAF during the handover,the session establishment request can also be initiated by the HOAF-CPto the HODF (i.e., the HOAF-CP initiates the performing of thehandover). However, in the two segmented modes shown in FIG. 3C, sincethe HOAF-CP is on the network side while the HODF is in the handover UE,the handover with the new connection initiated to establish by the HODFequals to a new session establishment adopting a mobile originationprocedure, and the handover with the new connection initiated toestablish by the HOAF-CP equals to a new session establishment adoptinga mobile termination procedure. Use mobile termination procedureincludes an additional step of inquiring the current position of theuser than the mobile origination procedure. In addition, when the HODFis located in the CS domain, a paging process is further needed.Therefore, the mobile termination procedure is generally slow thanmobile origination procedure. Although both the two methods can beadopted, the new connection is established more quickly when the HODFinitiates the establishment of the new connection, and therefore betterhandover effect can be achieved. So, in the following embodiments, newconnections initiated to establish by the HODF are taken as examples.

The segmented mode in the above two embodiments can be represented byFIG. 8. FIG. 8 is a schematic diagram illustrating a system of anIMS-IMS handover when access networks of the HOSF and HODF aredifferent. In FIG. 8, during the establishment of the initial sessionconnection, the connection between the HOAF-CP and the HOSF in thehandover UE through the access network 1 is the control plane firstconnection, i.e., leg1.

The connection between the HOAF-CP and the terminal side of the peeruser is leg2, and the handover user implements the conversation with thepeer user through the leg1 and the leg2. When the handover condition issatisfied, the HODF establishes a second connection, i.e., leg3, withthe HOAF-CP. When the handover is accomplished, the handover usercontinues the conversation with the peer user through the leg3 and theleg2.

During the IMS-IMS handover in various modes, a third user may initiatea call to the HOSF used by the handover user in the initial connection,wherein the call may be towards the shared IMPU of the HOSF and the HODFor towards the dedicated IMPU of the HOSF. Since in the embodiments ofthe present invention, the session handover process from the HOSF to theHODF may be invoked by various reasons, such as, the HOSF is about tolose the connection to the network, e.g., when the radio signaldeteriorates, or the battery of the handover UE is used up, so properactions should also be taken in such cases. In other words, the callfrom the third user to the HOSF should also be delivered to the HODF inthe handover UE according to the specific handover reason.

FIG. 6A is a flowchart illustrating a procedure of delivering a callfrom a third user to the HODF in the handover UE while the IMPU of thehandover user is shared by the HOSF and the HODF according to anembodiment of the present invention. As shown in FIG. 6A, taking theend-to-end/terminal-controlled mode as an example, when the HOSF and theHODF share the IMPU of the handover user while having different IMPIs,the procedure of delivering a call from a third user to the HODF in thehandover UE during the session handover includes the following steps:

Step 6101: the HOSF in the handover UE establishes a connection with theHOAF on the terminal side of the peer user to implement theconversation.

Step 6102: during the session handover, the home IMS network of thehandover user receives a session establishment request from a thirduser, and the request is towards the shared IMPU of the HOSF and theHODF of the handover user.

Step 6103: the home IMS network of the handover user performs a forkingprocess to the session establishment request following a standardprocedure, and sends the session establishment request to the HOSF andthe HODF respectively.

The forking process includes: according to corresponding legal contactaddresses of the IMPU, synchronously send or serially attempt to sendthe received session establishment request to multiple addresses basedon a preference settings of the user.

Step 6104: the HOSE returns a session refuse message to the home IMSnetwork of the handover user, the home IMS network of the handover userthen returns a session refuse acknowledgement to the HOSF.

Step 6105: the HODF determines a processing strategy for the twosessions, i.e., the handover session with the peer user and the newsession of the third user, according to pre-set local configurations anda user indication.

For example, the processing strategy of the HODF is to accept the newsession from the third user while holding the handover session. Then theHODF holds the handover session with the terminal side of the peer userafter the session handover, the HOAF releases the session between theHOAF and the HOSF. After that, the handover user establishes a newsession with the third user through the HODF and performs a medianegotiation interaction, and finally establishes the new session withthe third user.

Step 6106: after completing the session handover and holding thehandover session, the HOAF sends a session release message to the HOSF.

Step 6107: the HOSF sends a session release success message to the HOAF.

Thus the session between the HOSF and the HOAF is released. Step 6108:at the same time, the HODF establishes a session and performs a medianegotiation interaction with the third user, and finally sends a sessionanswer response to the third UE.

Step 6109: the third UE returns a session answer responseacknowledgement to the HODF, and the handover user establishes a sessionconnection with the third user through the HODF.

FIG. 6B is a flowchart illustrating a procedure of delivering a callfrom a third user to the HODF in the handover UE while the IMPU of thehandover user is not shared by the HOSF and the HODF according to anembodiment of the invention. As the HOSF and the HODF have differentIMPUs, a number modification process by a network control entity isrequired to deliver the call originally destined to the HOSF to theHODF. The network control entity can be the HOCF in thenetwork-controlled mode or the HOAF-CP in the segmented mode. In thepresent embodiment, the HOCF is taken as the network control entity. Asshown in FIG. 6B, taking the CP-segmented/network-controlled mode as anexample, when the HOSF and the HODF employ different IMPUs of thehandover user, the procedure of delivering a call from a third user tothe HODF in the handover UE during the session handover includes thefollowing steps:

Step 6201: during the establishment of the initial session, the HOSF inthe handover UE triggers the control function of the AS which performsthe functions of the HOCF/HOAF-CP in the home IMS network of thehandover user, and establishes the CP-segmented connections under thecontrol of the HOAF-CP to implement the conversation.

Step 6202: during the session handover, the home IMS network of thehandover user receives a session establishment request from a thirduser, wherein, the session establishment request is towards the IMPU ofthe HOSF of the handover user.

Step 6203: the HOCF in the home IMS network of the handover user learnsthat the new session from the third user should be delivered to the HODFin the handover UE according to the handover status and relevant servicelogic.

The handover status and the relevant service logic include: the HOCFlearns that the HOSF is about to lose the network connection accordingto the handover request sent by the HOSF.

Step 6204: the HOCF indicates to modify the destination identifier inthe session establishment request from pointing to the IMPU of the HOSFto the IMPU of the HODF, and then sends the modified sessionestablishment request to the HODF.

Step 6205: the HODF determines a processing strategy for the handoversession with the peer user and the new session of the third useraccording to the pre-set local configuration and the user indication.

For example, the processing strategy of the HODF is to refuse the newsession of the third user. Then the HODF returns a session refusemessage to the third user through the home IMS network of the handoveruser, the third user returns a session refuse confirmation to completethe release of the new session. And then the HODF continues with thesubsequent process of the handover session, e.g., establishment of thesession between the HODF and the HOAF-CP, the session re-negotiationbetween the HOAF and the terminal side of the peer user, and the finalrelease of the session between the HOAF-CP and the HOSF by the HOAF-CP,as illustrated in following steps.

Step 6206: the HODF sends a session refuse message to the third UEthrough the IMS network, and the third user returns a session refuseconfirmation to the HODF.

Step 6207: the HODF continues with the handover of the session with theterminal side of the peer user, establishes a session connection betweenthe HODF and the HOAF to continue the conversation.

Step 6208: the HOAF-CP sends a session release message to the HOSF, theHOSF then returns a session release success message to the HOAF-UP.

The CS-IMS handover is described hereinafter.

At this time, the correlated relationship between the HOSF and the HODFis the subscription identities of the same user in different networksubsystems, i.e., the MSISDN and the IMPU corresponding to the sameuser.

The CS-IMS handover can be accomplished in various service modes, asshown in FIGS. 3A, 3B and 3C, and the new connection can be initiated toestablish by the HODF or by the HOAF-CP. Only some typical embodimentsare described hereinafter.

FIG. 7A is a flowchart illustrating a CS-IMS handover in anend-to-end/terminal-controlled mode according to an embodiment of thepresent invention. As shown in FIG. 7A, taking the handover in theend-to-end/terminal-controlled mode while the new connection isinitiated to establish by the HODF as an example, the CS-IMS handover inthe embodiment of the present invention includes:

Step 7101: the HOSF in the handover UE initiates a call establishmentrequest in the CS domain to the terminal side of the peer user, and thecall establishment request is routed in the CS domain to the CS/IMSinterworking gateway MOCF/IM-MGW.

In the present embodiment, the peer UE is located in an IMS network.Therefore, the call establishment request is routed in the CS domain tothe CS/IMS interworking gateway MGCF/IM-MGW so as to enter the IMSnetwork of the peer user according to the existing called party analysisprocedure. However, when the peer UE is located in a CS/PSTN, in orderto implement the IMS based handover in various modes, the CS domainwhere the handover user is currently located needs to route the call tothe home IMS network of the handover user. Wherein, the CS domain candetermine the route to the MGCF/IM-MGW and further to the home IMSnetwork of the handover user according to the information including: CSrouting strategies (e.g., according to local static routingconfiguration, or triggering CAMEL for dynamic control by intelligentplatform according to various conditions), or Number Portability (NP)mechanisms, or analysis of the called party number, i.e., the number ofthe peer user side (the handover UE provides a called party number witha special prefix in the initiated CS call establishment request, and theCS domain where the handover user currently is located performs aspecial routing control according to the prefix).

Step 7102: the CS/IMS interworking gateway performs a CS/IMSinterworking process, sends an IMS session establishment request to theterminal side of the peer user through relevant nodes of the home IMSnetwork of the handover user, the home IMS network of the peer user andthe IMS network the peer user currently located;

Step 7103: perform a media negotiation and establish an IMS sessionbetween the terminal side of the peer user and the CS/IMS interworkinggateway.

Step 7104: the terminal side of the peer user returns an IMS sessionanswer response, the CS/IMS interworking gateway converts the IMSsession answer response into a CS call answer response and sent the CScall answer response to the HOSF; at the same time, the CS/IMSinterworking gateway returns an IMS session answer responseacknowledgement to the terminal side of the peer user, and establishes aCS/IMS interworking session between the HOSF and the terminal side ofthe peer user.

Step 7105: the HODF completes an IMS registration and sends a handoverevent notification to the HOSF, and then the HOSF returns an eventnotification confirmation to the HODF.

Step 7106: the HOSF determines whether the session handover condition iscurrently satisfied; if it is satisfied, the HOSF sends a handoverindication carrying session information to the HODF, then the HODFreturns a handover indication confirmation to the HOSF and executes Step7107, otherwise, the HOSF continues with the session and ends thecurrent procedure.

Step 7107: the HODF sends a session establishment request carrying thesession information and the handover indication to the HOAF-CP.

Step 7108: the HOAF-CP determines whether the session establishmentrequest is legal; if the request is legal, execute Step 7109; otherwise,end the current procedure.

Step 7109: the HOAF and the HODF perform the establishment of the newsession and the media negotiation during the session establishment.

Step 7110: the HOAF returns a session answer response to the HODF.

Step 7111: the HODF returns a session answer response acknowledgement tothe HOAF.

Thus, the new session connection between the HODF and the HOAF isestablished.

Step 7112: the HOAF sends a session release message in the IMS networkto the CS/IMS interworking gateway MGCF/IM-MGW.

Step 7113: the CS/IMS interworking gateway performs a CS/IMSinterworking process and sends a call release message in the CS domainto the HOSF.

Step 7114: the HOSF sends a call release complete message to the CS/IMSinterworking gateway MGCF/IM-MGW.

Step 7115: the interworking gateway MGCF/IM-MGW performs a CS/IMSinterworking process and sends a call release confirmation to the HOAF.

Thus, the end-to-end session handover is complete, the handover user andthe peer user continue their conversation through the new sessionconnection between the HODF and the HOAF.

To further illustrate the effectiveness of the scheme of the presentembodiment in a CS-IMS handover when the peer user is in the CS/PSTN, anembodiment of the present invention is described hereinafter, whereinthe handover is in the CP-segmented/terminal-controlled mode and theHODF initiates the performing of the handover, and the peer UE iscurrently located in a CS network.

FIG. 7B is a flowchart illustrating a CS-IMS handover in aCP-segmented/terminal-controlled mode when a peer user is also locatedin a CS domain according to an embodiment of the invention. As shown inFIG. 7B, taking the handover in the CP-segmented/terminal-controlledmode and the performing of the handover initiated by the HODF as anexample, the CS-IMS handover in the embodiment of the present inventionincludes:

Step 7201: the HOSF in the handover UE initiates a call establishmentrequest in the CS domain to the terminal side of the peer user; the callestablishment request is routed by the CS domain where the handover useris currently located to the CS/IMS interworking gateway MGCF/IM-MGWinterworking with the home IMS network of the handover user.

Step 7202: the CS/IMS interworking gateway performs a CS/IMSinterworking process, the control function in the AS which performs thefunctions of the HOAF-CP is triggered in the home IMS network of thehandover user, and the HOAF-CP controls the establishment of thesegmented connection. Then after an interworking processing and CSdomain routing, the CS call establishment request which is used toestablish the SEG2 between the HOAF-CP and the terminal side of the peeruser is sent to the terminal of the peer side according to the standardinterworking procedure and through an interworking gateway, which isselected by the IMS network and interworking with the CS domain wherethe peer UE is currently located.

The home IMS network of the handover user may trigger the controlfunction in the AS that performs the functions of the HOAF-CP followingthe current IMS standards according to an E.164 number or a specialprefix of the E.164 number modified in the routing control of the CSdomain where the handover user is currently located, e.g., taking aspecial E.164 number or a prefix as an IMS domain Public ServiceIdentity (PSI).

As described above, in the present embodiment, the peer UE is currentlyin the CS domain, so the terminal side mentioned herein actuallyincludes the peer UE, the interworking gateway interworking for the peerUE, and the CS connection between the interworking gateway and the peerUE.

Step 7203: the HOAF-CP establishes the segmented IMS session andperforms a media negotiation during the session establishmentrespectively with the terminal side of the peer user (actually theinterworking gateway interworking with the CS domain where the peer UEis currently located) and the interworking gateway interworking with theCS domain where the handover user is currently located.

Step 7204: the peer UE returns a CS call answer response, aninterworking gateway corresponding to the peer UE converts the CS callanswer response into an IMS session answer response and sends the IMSsession answer response to the HOAF-CP; the HOAF-CP then returns the IMSsession answer response to the CS/IMS interworking gateway, the CS/IMSinterworking gateway converts the IMS session answer response into a CScall answer response and sends the CS call answer response to the HOSF;meanwhile, the CS/IMS interworking gateway sends an IMS session answerresponse acknowledgement to the HOAF-CP, which in turn sends the IMSsession answer response acknowledgement to the terminal side of the peeruser, thus the two segments of the CS/IMS interworking sessions areestablished respectively from the HOSF to the HOAF-CP and from theHOAF-CP to the terminal side of the peer user; then the handover userand the peer user implements conversation through the session segments.

Step 7205: the HODF completes an IMS registration.

Step 7206: the HODF performs an internal interaction with the HOSF tocomplete handover condition detection, and indicates to initiate ahandover.

Step 7207: the HODF sends a session establishment request carryingsession information and a handover indication to the HOAF-CP.

Step 7208: the HOAF-CP judges whether the session establishment requestis legal; if it is legal, execute Step 7209, otherwise, end the currentprocedure.

Step 7209: the HOAF-CP sends a re-negotiation request to the terminalside of the peer user, and indicates the terminal side of the peer userto re-direct the media streams to the HODF.

Step 7210: the terminal side of the peer user returns a re-negotiationresponse to the HOAF-CP, which in turn returns a session answer responseto the HODF.

Step 7211: the HODF returns a session answer response acknowledgement tothe HOAP-CP, which in turn returns a re-negotiation responseacknowledgement to the terminal side of the peer user.

Thus the new session connection between the HODF and the HOAF-CP isestablished, and the media streams are re-directed through there-negotiation between the HOAF-CP and the terminal side of the peeruser; the peer user and the handover user are transferred to the newconnection to continue their conversation.

Step 7212: the HOAF sends a session release message in the IMS networkto the CS/IMS interworking gateway MGCF/IM-MGW.

Step 7213: the CS/IMS interworking gateway performs CS/IMS interworkingprocess and sends a call release message in the CS domain to the HOSF.

Step 7214: the HOSF sends a call release complete message to the CS/IMSinterworking gateway MGCF/IM-MGW.

Step 7215: the interworking gateway MGCF/IM-MGW performs a CS/IMSinterworking process and sends a session release confirmation to theHOAF.

Then, the handover from IMS to CS domain is described hereinafter.

Similarly, the IMS-CS handover can be accomplished in various servicemodes, as shown in FIGS. 3A, 3B and 3C, wherein, the new connectionestablished during the handover procedure can be initiated to establishby the HODF or by the HOAF-CP. The embodiment hereinafter shows only theperforming of the handover initiated by the HODF in theCP-segmented/network-controlled mode.

FIG. 9 is a schematic diagram illustrating a system implementing anIMS-CS handover in a CP-segmented mode.

In FIG. 9, during the establishment of the initial session connection,the connection between the HOSF in the handover UE and the HOAF-CP isthe control plane first connection, i.e., leg1, the connection betweenthe HOAF-CP and the terminal side of the peer user is leg2, and thehandover user performs a conversation with the peer user through theleg1 and the leg2. When the handover condition is satisfied, the HODF inthe handover UE establishes a second connection, i.e., leg3, with theHOAF-CP through the CS domain and the interworking gateway. When thehandover is accomplished, the handover user continues to performconversation with the peer user through the leg3 and the leg2.

FIG. 7C is a flowchart illustrating an IMS-CS handover in aCP-segmented/network-controlled mode according to an embodiment of thepresent invention.

As shown in FIG. 7C and FIG. 9, taking the handover in theCP-segmented/network-controlled mode and the performing of the handoverinitiated by the HODF as an example, the IMS-CS handover according tothe embodiment of the present invention includes:

Steps from Step 7301 to Step 7304 are basically identical to Step 5301,wherein the AS that performs the function of the HOAF-CP also performsthe function of the HOCF.

Step 7305: the HOCF detects that the handover condition is satisfied,and sends a handover indication to the HOSF.

The HOCF may judge whether the handover condition is satisfied accordingto the user pre-configured strategies and related conditions, or afterreceiving a notification that the HOSF is about to lose the networkconnection or a notification that the HODF is successfully connected tothe network, or according to the user indication from the HOSF/HODF. Atthe same time, the HOCF may initiate the handover through directlyinstructing the HOAF to initiate the performing of the handover, orthrough directly instructing the HODF to initiate the performing of thehandover, or through sending a notification to the HOSF and the HOSFinstructing the HOAF/HODF to initiate the performing of the handover. Inthe present embodiment, the HOCF judges according to the userpre-configured strategies and related conditions, and instructs the HODFthrough the HOSF to initiate the handover.

As described above, the handover may also be detected and initiated bythe HOSF or the HODF.

Step 7306: the HOSF instructs the HODF through the internal interactionto initiate a handover, and provides original session information to theHODF, the HODF sends a CS call establishment request in the CS domainwhere the HODF is located to the HOAF-CP, wherein, the CS callestablishment request is routed by the CS domain to the CS/IMSinterworking gateway interworking with the home IMS network of thehandover user, converted into an IMS session establishment request bythe CS/IMS interworking gateway, and routed by the home IMS network ofthe handover user to the AS that performs the functions of the HOAF-CP.

Since it is the HODF who initiates the performing of the handover, thecall establishment request needs to carry a handover indication,original session information and a location identifier of the HOAF-CP.In the present embodiment, since the HOAF-CP is located on the homenetwork side of the handover user, so the HODF can obtain the locationidentifier of the HOAF-CP from manual settings pre-set by the user orfrom network settings issued to and saved in the HODF. At the same time,the HODF obtains the original session information from the HOSF.Therefore the HODF can combine the location identifier and the sessioninformation according to a pre-configured rule to form a special E.164number. What is pointing to the AS that performs the function of theHOAF-CP can be a wildcarded IMS domain PSI, i.e., a PSI allowable withan extended part. The PSI with any value in the extended part, i.e.,distinct PSI, matches the wildcarded PSI, and this extended part can beused to carry information to be transmitted. Therefore the IMS networkcan restore a distinct PSI from the special E.164 number through thestandard procedure, and routes the session establishment request to theAS corresponding to the PSI (i.e., the AS that performs the function ofthe HOAF-CP), the AS compares this distinct PSI with the wildcarded PSIand obtains the information corresponding to the extended pail, i.e.,the original session information. In addition, the PSI itself may serveas a handover indication.

The call establishment request may also use an ISUP special IE, e.g., auser-to-user information IE, or a call reference number IE, or asub-address IE, to carry the original session information. At this time,the interworking gateway needs to convert the special IE to anindication in the IMS session establishment request to transmit to theHOAF-CP.

When the HOAF-CP initiates the performing of the handover, the HOAF-CPcan uses an E.164 number pointing to the HODF, such as the MSISDN of theHODF, as a destination identifier in the session establishment request,and sends the session establishment request to the HODF through the IMSnetwork, the CS/IMS interworking gateway MGCF/IM-MGW and each entity inthe CS domain.

The E.164 number pointing to the HODF may be the MSISDN of the user orthe E.164 number with a special prefix. And the E.164 number can beprovided by the HOSF or the HODF in the handover indication to theHOAF-CP, or be obtained through deduction by the HOAF-CP from theidentifier of the HOSF according to the network numbering plan.

Step 7307: the HOAF-CP verifies whether the session establishmentrequest carries a handover indication, and further verifies whether thesession establishment request is legal according to the sessioninformation in the request; if it is legal, execute Step 7308,otherwise, end the current procedure.

Step 7308: the HOAF-CP sends a re-negotiation request through the SEG2between the HOAF-CP and the terminal side of the peer user to initiatethe re-negotiation, implements the media re-direction and modifies themedia attribute of the SEG2 according to the media negotiation result onthe SEG3 between the HOAF-CP and the HODF.

Step 7309: the peer UE returns a re-negotiation response, the HOAF-CPthen returns an IMS session answer response to the CS/IMS interworkinggateway, the CS/IMS interworking gateway converts the IMS session answerresponse into a CS call answer response and sent the CS call answerresponse to the HODF through the CS domain.

Step 7310: the CS/IMS interworking gateway returns an IMS session answerresponse acknowledgement to the HOAF-CP, which in turn returns are-negotiation response acknowledgement to the terminal side of the peeruser.

The new connection between the HODF and the HOAF-CP is thus established,and the media streams on the terminal side of the peer side isre-directed to the HODF through the re-negotiation initiated under thecontrol of the HOAF-CP; therefore the handover user and the peer usercontinues their conversation through the new connection.

Step 7311: the HOAF-CP sends a session release message to the HODFthrough the IMS network.

Step 7312: the HOSF returns a session release confirmation message tothe HOAF-CP, and the HOSF and the HOAF release the session connectionbetween them.

In the above embodiments, in order to keep the continuity of the mediaexchange during the handover better, the HOAF-UP can be equipped withmedia stream duplication and filtering functions. The HOAF-UP duplicatesthe media stream to the terminal side of the handover user and sends theduplicated media streams to the HOSF and the HODF simultaneously duringthe handover, and selects a valid part from the media streams from theHOSF and the HODF to send to the terminal side of the peer user (in casethat the HOAF-UP is located on the home network side of the handoveruser in the CPandUP-segmented mode) or to present to the user (in casethat the HOAF-UP is located on the terminal side of the peer user in amode other than the CPandUP-segmented mode).

To conclude, the method for maintaining session continuity in a networkthat supports multiple access technologies includes the following keypoints:

1. The trigger of the HOCF function in the network-controlled modeand/or the trigger of HOAF-CP function in the segmented mode during thesession establishment:

(1) during the initial session establishment, according to the generalservice triggering method in the IMS, the service node S-CSCF assignedto the handover user by his/her home IMS network triggers the session tothe AS that performs the functions of the HOCF/HOAF-CP according to theiFC in the subscription data downloaded from the HSS during the userregistration. Wherein, in the segmented/network-controlled mode, theHOAF-CP and the HOCF are implemented in the same AS. At this time, inorder to reduce the influence to other IMS services, the HOAF-CP shouldbe set as close as possible to the HOSF/HODF, in other words;

when the HOSF is the calling party in the initial session establishment,the priority level of the iFC corresponding to the AS that performs thefunction of the HOCF/HOAF-CP should be set as high as possible;

when the HOSF is the called party in the initial session establishment,the priority level of the iFC corresponding to the AS that performs thefunction of the HOCF/HOAF-CP should be set as low as possible;

(2) when the HOSF and the HODF correlated with each other throughsharing an IMPU, since the IMS user subscription data is identified onlythrough the user public identity, and the services have been triggeredon the connection between the HOAF-CP and the terminal side of the peeruser as much as possible in the segmented mode, duplicate triggering ofother ASs on the new session between the HOAF and the HODF should beavoided. Besides the iFC priority level settings, other adoptablemethods include the following: identifying the new session establishedduring the handover when the IMS service node (S-CSCF) assigned to thehandover user performing the iFC evaluation, and so as avoid the servicetriggering. Wherein, the method for identifying the new session mayinclude, but is not limited to, the following: identifying the newsession according to the handover indication in the session establishinformation, or according to special calling party information and/orcalled party information (e.g., the called party information pointing tothe HOAF when the HODF initiates the operation of the handover, thecalling party information that represents the HOAF when the HOAFinitiates the performing of the handover) in the session establishinformation.

2. The handover condition detection:

(1) the handover condition detection can be performed by the HOSF,according to the conditions including the following cases and anycombination of them; it is detected that the local end is about to losethe connection to the IMS network due to the problem of the accessnetwork or local equipment; a notification indicating that the HODF hassuccessfully connected to the IMS is received; judgment based on userpre-configured conditions or related conditions; judgment based on theuser indication; it is detected that the communication quality of thecurrent access point of the HODF is higher than that of the local end;the priority level of the operator of the HODF is higher than that ofthe local end; the charging rate of the current access method of theHODF is lower than that of the local end;

(2) the handover condition detection can also be performed by the HODF,according to the conditions including the following cases and anycombination of them: the local end has successfully connected to theIMS; a notification indicating that the HOSF is about to lose theconnection to the IMS network is received; judgment based on userpre-configured strategies and related conditions; judgment based on theuser indication: the communication quality of the local end is higherthan that of the HOSF; the priority level of the operator of the localend is higher than that of the HOSF; the charging rate of current accessmethod of the local end is lower than that of the HOSF;

(3) the handover condition detection can also be performed by the HOCF,according to the conditions including the following cases and anycombination of them: judgment based on user pre-configured strategiesand related conditions; a notification indicating that the HOSF is aboutto lose the connection to the IMS network is received; a notificationindicating that the HODF has successfully connected to the IMS isreceived; judgment based on the user indication from the HOSF/HODF; thecommunication quality of the access point of the HODF is higher thanthat of the HOSF; the priority level of the operator of the HODF ishigher than that of the HOSF; the charging rate of the current accessmethod of the HODF is lower than that of the HOSF.

3. the handover authorization by the HOCF, can be performed according tothe conditions including the following items and any combination ofthem: the legality of the HODF; the subscription capability of the HODF;the correlated relationship between the HOSF and the HODF; userpre-configured strategies and related conditions; network pre-configuredstrategies and related conditions; the current status and attribute ofthe session between the HOSF and the HOAF.

4. the handover initiation, including the handover request and thehandover authorization, which can summarized as:

(1) the handover condition detection and the handover initiation areperformed by the HOSF, including: the HOSF directly instructs the HOAFto initiate the performing of the handover; or the HOSF directlyinstructs the HODF to initiate the performing of the handover; or theHOSF sends a request to the HOCF, which sends an indication after theauthorization succeeds to indicate the HOAF to initiate the performingof the handover; or the HOSF sends a request to the HOCF, which sends aindication after the authorization succeeds to indicate the HODF toinitiate the performing of the handover, or the HOSF sends a request tothe HOCF, and instructs the HOAF to initiate the performing of thehandover after the HOCF returns a confirmation when the authorizationsucceeds; or the HOSF sends a request to the HOCF, and instructs theHODF to initiate the performing of the handover after the HOCF returns aconfirmation when the authorization succeeds;

(2) the handover condition detection and the handover initiation areperformed by the HODF, including: the HODF directly initiates theperforming of the handover; or the HODF sends a request to the HOCF,which sends an indication after the authorization succeeds to indicatethe HOAF to initiate the performing of the handover; or the HODF sends arequest to the HOCF, which returns a confirmation after theauthorization succeeds to indicate the HODF to initiate the performingof the handover;

(3) the handover condition detection and the handover initiation areperformed by the HOCF, including: the HOCF handover; or HOCF directlyinstructs the HODF to initiate the performing of the handover; or theHOCF informs the HOSF, which instructs the HOAF to initiate theperforming of the handover; or the HOCF informs the HOSF, whichinstructs the HODF to initiate the performing of the handover;

(4) during the handover initiation, when an entity sends a handoverevent notification or initiates a handover request to another entity,the interaction between them includs, but is no limited to, thefollowing: when the two entities (e.g., the HOSF and HODF, or the HOCFand the HOAF-CP) are co-located in the same entity, internal customizedinteractions can be adopted, e.g., interactions through internalmessages, or through a shared internal memory; otherwise, theinteraction can be performed through SIP INFO and SIP NOTIFY message; orthough service location means such as SLP;

(5) during the handover initiation, when an entity instructs anotherentity to perform the handover, the interaction between them includes,but no limited to, the following: when the two entities (e.g., the HOSFand HODF, or the HOCF and the HOAF-CP) are co-located in the sameentity, internal customized interactions can be adopted, e.g.,interactions through internal messages, or through a shared internalmemory; otherwise, the interaction can be performed through SIP INFO orSIP NOTIFY message, with the information of the other entity in themessage; or through SIP REFER message, with the information of the otherentity in a REFER-TO header.

5. the handover execution:

(1) the performing of the handover is initiated by the HODF;

The HODF initiates a session establishment request to the HOAF-CP andinstructs the HOAF-CP to replace the original session connection to theHOSF when the new session is established; the session establishmentrequest carries the information of the session to be replaced betweenthe HOSF and the HOAF and a handover indication; the handover indicationcan also be implicitly represented by other specific handover-relatedinformation.

In the session establishment request, the new session can be indicatedto be a replacement of the original session between the HOSF and theHOAF and session related information may be carried in the sessionestablishment request through the REPLACES header field or specialdestination identifier;

the session establishment request is pointing to the HOAF-CP, and thelocation identifier pointing to the HOAF-CP is obtained from thefollowing: the information carried by the handover indication of theHOCF/HOSF during the handover initiation; the information carried in thehandover or handover-related event notification from the HOCF/HOSFduring the handover condition detection; wherein, the last two means,i.e., local pre-configured settings or locally stored settings issued bythe network, is applicable in the CP-segmented mode and theCPandUP-segmented mode.

The network entities involved between the HOAF and HODF can performspecial processes according to the handover indication carried in thesession establishment request, the special processes include: avoidingduplicate triggering of service or avoiding the influence from otherservices, or special charging process, etc.

The HOAF verifies the legality of the session establish requestaccording to the handover indication and initial session information inthe session establishment request, establishes the new session, replacesthe original session with the new session, and releases the originalsession; wherein, the replacement of the original session includes, butis not limited to: re-directe the media streams exchanged with the HOSFin the original session to exchanged with the HODF and accomplish themodification of the media streams required by the difference of themedia capabilities between the HODF and the HOSF through the sessionmedia negotiation.

The HOAF may perform some special processes according to the handoverindication in the session establishment request, including avoidringing.

(2) the performing of the handover is initiated by the HOAF-CP;

The HOAF initiates a session establishment request to the HODF andreplaces the original session connection to the HOSF when the newsession is established; the session establishment request can furthercarry a handover indication and HOSF-related information; wherein, thehandover indication can also be implicitly carried by other specifichandover-related information.

After receiving the session establishment request, the HODF interactswith the HOAF-CP to establish the new session, and during the sessionestablishment, the HODF performs a media negotiation with the HOAF-CPaccording to the media capability of the local end.

The session establishment request is pointting to the HODF, and thelocation identifier pointing to the HODF can be obtained from itemsincluding: the information carried by the handover indication of theHOCF/HOSF during the handover initiation.

The HODF can verifies the legality of the session establishment requestaccording to the HOSF-related information in the request.

The HODF can perform some special processes according to the handoverindication or the HOSF-related information in the session establishmentrequest, including avoid ringing.

The network entities involved between the HOAF and the HODF can performspecial processes according to the handover indication and theHOSF-related information carried in the session establishment request,the special processes include avoid duplicate triggering of service, oravoid the influence from other services, or special charging process,etc.

It should be noted that when both the HOSF and the HODF register in theIMS (in other words, there is no such cases that access the IMS througha CS call), based on the IMS subscribed association between the HOSF andthe HODF, a same service node is guaranteed to be assigned to the twoentities according to the standard service node assignment processduring the IMS network user registration.

6. Special processes correlated with the CS domain:

The CS call, which is converted into an IMS session by the CS/IMSnetwork, is regarded as a special IMS access method, and when theHODF/HOSF adopts such access method, the IMS-CS handover will beaccomplished through the above IMS-IMS handover method, including:

(1) the HODF and the HOSF correspond respectively to the subscriptionidentities of the same user in different network subsystems, in otherwords, they correspond respectively to the MSISDN and the IMPU of thesame user;

(2) when the HOSF corresponds to the above CS-IMS call, the CS domainroutes the call to the CS/IMS interworking gateway MGCF/IM-MGW, andestablishes a CS-IMS interworking call after the conversion performed bythe MGCF/IM-MGW; the CS domain can decide the route according to theconditions including:CS routing strategies, NP mechanism and the calledparty number analysis;

(3) when the HODF corresponds to the above CS-IMS call, during thesession establishment between the HODF and the HOAF-CP, the CS-IMSinterworking call converted by the MGCE/IM-MGW is established throughthe following process:

when the performing of the handover is initiated by the HODF, the HODFtakes the E.164 number which points to the HOAF-CP as the called partynumber; prefix may be added to distinguish the E.164 number from otherE.164 numbers, or the E.164 number can be a special E.164 number.

In the end-to-end mode, the number is provided by the HOSF or the HOCF;the number can be the MSISDN of the peer user, or the E.164 number inthe IMPU of the peer user in the Tel-URI format, or the E.164 number inthe IMPU of the peer user in the Tel-URI format with a special prefix;

in the segmented mode, the number is provided by the HOSF or the HOCF,or set as a uniform service access number in the UE, or issued by thenetwork and stored in the UE at any prior time (e.g., when the userregisters in the IMS or updates the location in the CS domain); and thespecial E.164 number or the prefix of the E.164 number may correspond toa PSI, therefore the session can be routed to the AS that performs thefunction of the HOAF-CP in a standard termination routing method for thePSI.

Moreover, the CS call establishment request from the HODF further needsto carry the original session information, the HODF may use the calledparty number to carry the original session information, e.g., in thesegmented mode, the identifier pointing to the AS that performs thefunctions of the HOAF-CP can be a wildcarded PSI, the HODF combines theidentifier pointing to the AS and the original session information toform a special E.164 number according to pre-configured rules. Thus theIMS network can recover a distinct PSI from the E.164 number, and thenroute the session establishment request to the AS, which compares thedistinct PSI with the wildcarded PSI and to obtain the original sessioninformation.

the original session information can also be carried by a special IE inthe ISUP, e.g., a use-to-user information IE, or a call reference numberIE, or a sub-address IE, at this time, the CS/IMS interworking gatewayMGCF/IM-MGW needs to recover the original session information from thereceived CS call establishment request, and sends the original sessioninformation to the HOAF-CP through the converted IMS sessionestablishment request.

The CS domain is responsible for routing a CS call to the CS/IMSinterworking gateway MGCF/IM-MGW according to the E.164 number, andfurther establishes a CS-IMS interworking call converted by theMGCF/IM-MGW; the CS domain can decide the route according to theconditions including:CS routing strategies, NP mechanism and the calledparty number analysis.

The CS domain can perform, according to the E.164 number or the prefixof the E.164 number, some special processes including: avoid influencesof the supplementary services on the CS originating side, avoidCustomized Application for Mobile network Enhanced Logic (CAMEL)triggering, special charging process, including routing control forrouting the call to the CS/IMS interworking gateway.

The IMS network routes the session to the HOAF-CP according to thespecial E.164 number or the prefix of the E.164 number cooperating withthe MGCF, ENUM server and the data settings of the IMS network, wherein,the HOAF-CP includes the peer UE that performs the functions of the HOAFin the end-to-end mode and the AS that performs the function of theHOCF/HOAF-CP in the segmented mode.

When the performing of the handover is initiated by the HOAF-CP, theHOAF-CP may take the MSISDN of the handover user or the MSISDN numberwith a special prefix as the called party number, wherein, the MSISDNnumber is provided by the HOSFIHOCF in the handover indication, ordeduced from the identity of the HOSF according to the network numberingplan.

The IMS network mutes the session to the selected MGCF according to thenumber in combination with the data settings of the ENUM and IMSnetwork, the CS network accomplishes a standard number analysis and thecalled party addressing to route the call request to the HODF, andfurther establishes an IMS-CS interworking call converted by theMGCF/IM-MGW.

If the called party number is the MSISDN with a special prefix, thegateway can remove the prefix and uses the MSISDN to complete theinteraction with the CS network;

if the called party number is the MSISDN with a special prefix, thegateway can further recognize the call as an IMS-CS handover callaccording to the prefix after removing the prefix, so that a specialindication can be added into the CS call control signaling to enable theCS network or the peer user to perform a special process on the IMS-CShandover call;

The CS network can perform some special processes according to the abovespecial indication or prefix, including:

special charging process, e.g., reduce a part of CS call fee accordingto the indication, which needs corresponding function extension of thecharging system;

service conflict process, e.g., avoid the invocation of a part of CSdomain supplementary services according to the indication, such as callforwarding, call barring and intelligent triggering, which needscorresponding function extension of the CS network.

The HODF can perform some special processes according to the specialindication, including: identify the call as an IMS-CS handover call toprompt the user through a pop-up menu or special ringing, or avoidringing.

7. the processing of the session establishment request towards the IMPUof the HOSF from a third user;

(1) when the HOSF and the HODF are correlated through a shared IMPUwhich is used for the terminating addressing:

the IMS network performs a forking process on the correspondingaddresses of the HOSF and the HODF according to the IMPU;

the HOSF receives the forking session establishment request, and refusesthe request;

the HODF receives the forking session establishment request, regards therequest as a legal new session establishment request, and accepts orrefuses the request according to the new session processing principlesof the user/local end.

(2) when the HOSF and the HODF have different IMPUs which are used forthe terminating addressing in the network-controlled mode (including theend-to-end mode and segmented mode) or the segmented mode (including theCP-segmented mode and the CPandUP segmented mode), since all theterminating procedures towards the user will be triggered to the AS thatperforms the function of the HOCF/HOAF-CP, the AS can control there-direction of the session that originally destined to the IMPU of theHOSF to the IMPU of the HODF according to the handover status and theuser subscription. The above control can be implemented throughreturning a 3XX (SIP re-direction indication) or directly modifying aforwarding SIP message Request-URI (Universal Resource Identity, alsoreferred to as a destination identifier or a destination addressherein).

The fore-going are the implementations of the present invention in theIMS network, and when implemented in the NGN, the principles arebasically identical to those in the IMS network, and the differences inthe NGN are:

(1) the correlated relationship between the HOSF and the HODF:

in the NGN, a user does not have a clear definition of the IMPI and theIMPU, therefore in the NGN, the correlated relationship between the HOSFand the HODF is generally represented by a subscription associationbetween them on the service layer, or represented by an associationbetween other user identites similar as the IMPI and the IMPU.

(2) when the HOCF and the HOAF-CP are located on the network side of thehandover user, the mapping of their functions in the network and thetrigger of their functions during the initial session establishment:

in the NGN, there is no clear definition similar as the open servicearchitecture and service triggering control mechanism in the IMSnetwork. Therefore, in the NGN, the entities that performs the functionsof the HOCF/HOAF-CP on the network side in the network-controlled and/orsegmented mode can be a soft switching entity in the NGN, or an ASimplemented using the service architecture in the IMS for reference. Ifthe AS is adopted, the triggering of the AS can be performed through acustomized service triggering method defined in the NGN, or through amechanism similar to the IMS network triggering mechanism.

Despite of the above differences, the present invention is totallyapplicable to the NGN that supports multiple access technologies andadopts the SIP as the session control protocol.

To sum up, the foregoing are only the preferred embodiments of thepresent invention and are not for use for confining the protection scopeof the present invention. Any modification, equivalent substitution,improvement within the spirit and principle of the present inventionshould be covered in the protection scope of the present invention.

What is claimed is:
 1. A method for maintaining session continuity,comprising: triggering a Handover Anchor Function-Control Plane(HOAF-CP) on a home network side of a first user, and establishing afirst connection, wherein the first connection comprises a control planefirst connection between a Handover Source Function (HOSF) of the firstuser and the HOAF-CP and a user plane first connection between the HOSFand a Handover Anchor Function-User Plane (HOAF-UP) of a second user;establishing a control plane connection between the HOAF-CP and aterminal side of the second user, and establishing a session between thefirst user and the second user through the user plane first connection,the control plane first connection and the control plane connectionbetween the HOAF-CP and a terminal side of the second user; determiningthat a handover condition is satisfied, and establishing a secondconnection, wherein the second connection comprises a control planesecond connection between a Handover Destination Function (HODF) of thefirst user and the HOAF-CP and a user plane second connection betweenthe HODF and the HOAF-UP; and continuing the session between the firstuser and the second user through the user plane second connectionbetween the HODF and the HOAF-UP, the control plane second connectionbetween the HODF and the HOAF-CP, and the control plane connection,which is already established between the HOAF-CP and the terminal sideof the second user before the handover condition is satisfied.
 2. Themethod according to claim 1, further comprising: configuring a HandoverControl Function (HOCF) on the home network side of the first user,wherein the first connection and the second connection are establishedunder control of the HOCF.
 3. The method according to claim 2, whereinif it is the HOSF that detects the handover condition is satisfied, themethod comprising at least one of: the HOSF sending a handover requestto the HOCF, the HOCF performing an authorization and notifying the HOSFafter the authorization succeeds; after receiving an authorization passmessage from the HOCF, the HOSF sending a handover indication message tothe HODF or the HOAF-CP, and the HODF or the HOAF-CP which receives thehandover indication message initiating to establish the secondconnection; or, the HOSF sending a handover request to the HOCF, theHOCF performing an authorization and sending a handover indicationmessage to the HODF or the HOAF-CP if the authorization succeeds, andthe HODF or the HOAF-CP which receives the handover indication messageinitiating to establish the second connection.
 4. The method accordingto claim 2, wherein if it is the HODF that detects that the handovercondition is satisfied, the method comprising at least one of: the HODFsending a handover request to the HOCF, the HOCF performing anauthorization and if the authorization succeeds, the HOCF sending ahandover indication message to the HODF or the HOAF-CP, and the HODF orthe HOAF-CP, which receives the handover indication message, initiatingto establish the second connection; or the HODF sending a handoverrequest to the HOCF, the HOCF sending a handover indication message tothe HOSF and the HOSF sending the handover indication message to theHODF or the HOAF-CP; and the HODF or the HOAF-CP, which receives thehandover indication message, initiating to establish the secondconnection.
 5. The method according to claim 2, wherein if it is theHOCF that detects the handover condition is satisfied, the methodcomprising at least one of: the HOCF sending a handover indicationmessage to the HODF or the HOAF-CP, and the HODF or the HOAF-CP, whichreceives the handover indication message, initiating to establish thesecond connection; or the HOCF sending a handover indication message tothe HOSF and the HOSF sending the handover indication message to theHODF or the HOAF-CP, and the HODF or the HOAF-CP, which receives thehandover indication message, initiating to establish the secondconnection.
 6. The method according to claim 2, wherein the HOSF and theHODF of the first user have different IP Multimedia public identities(IMPUs), and the method comprises: a home network of the first userreceiving a session establishment request to the HOSF of the first userfrom a third user; the HOCF or the HOAF-CP on the home network side ofthe first user modifying a destination identifier pointing to the HOSFin the session establishment request to the one pointing to the HODF,and then the home network of the first user sending the sessionestablishment request to the HODF; the HODF determining whether toaccept the session establishment request from the third user.
 7. Themethod according to claim 2, wherein the HOCF is implemented in a sameApplication Server (AS) with the HOAF-CP.
 8. The method according toclaim 2, further comprising: the HOCF updating the handover conditioncorrelated with a terminal and pre-stored in the HOCF through signalinginteraction between the HOCF and the terminal.
 9. The method accordingto claim 2, wherein the HODF initiates to establish the secondconnection and the method further comprises: the HODF acquiring anidentifier pointing to the HOAF-CP and initiating a connectionestablishment request according to the identifier pointing to theHOAF-CP; wherein the identifier pointing to the HOAF-CP is obtained bythe HOSF during the establishment of the first connection and sent tothe HODF by the HOSF, or the identifier pointing to the HOAF-CP isobtained by the HOCF during the control of the establishment of thefirst connection and sent to the HODF by the HOCF.
 10. The methodaccording to claim 2, wherein the HODF initiates to establish the secondconnection and the method further comprises: the HODF obtaining sessioninformation of the first connection and initiating a connectionestablishment request to the HOAF-CP carrying the session information ofthe first connection; wherein the session information of the firstconnection is obtained by the HOCF during the control of theestablishment of the first connection and sent to the HODF by the HOCF.11. The method according to claim 2, wherein the HOAF-CP initiates toestablish the second connection and the method further comprises: theHOAF-CP obtaining an HODF identifier pointing to the HODF and initiatinga connection establishment request to the HODF according to the HODFidentifier; wherein the HODF identifier is sent to the HOAF-CP by theHOSF or the HODF identifier is obtained by the HODF during the controlof the establishment of the first connection and sent to the HOAF-CP bythe HOCF.
 12. The method according to claim 1, wherein if it is the HOSFthat detects the handover condition is satisfied, the HOSF sending ahandover indication message to the HODF, and the HODF initiating toestablish the second connection; or the HOSF sending a handoverindication message to the HOAF-CP, and the HOAF-CP initiating toestablish the second connection; or if it is the HODF that detects thehandover condition is satisfied, the HODF initiating to establish thesecond connection.
 13. The method according to claim 12, wherein if itis the HOSF that detects the handover condition is satisfied, the HOSFdetecting that a communication quality by using the HODF is higher thana communication quality by using the HOSF.
 14. The method according toclaim 1, wherein the step of establishing the second connectioncomprises: the HODF obtaining session information of the firstconnection and initiating a connection establishment request to theHOAF-CP, wherein the connection establishment request carries thesession information of the first connection and a handover indication;after receiving the connection establishment request, the HOAF-CPinteracting with the HODF to establish the control plane secondconnection between the HODF and the HOAF-CP, and the user plane secondconnection between the HODF and the HOAF-UP.
 15. The method according toclaim 14, further comprising: before the HOAF-CP interacts with theHODF, the HOAF-CP checking whether the connection establishment requestcarries the handover indication; if the connection establishment requestcarries the handover indication, the HOAF-CP checking whether theconnection establishment request is legal according to the sessioninformation carried in the connection establishment request; if theconnection establishment request is legal, the HOAF-CP executing thestep of interacting with the HODF to establish the control plane secondconnection and the user plane second connection, if the connectionestablishment request is illegal, the HOAF-CP sending a message to theHODF to reject the connection establishment request, keeping the sessionwith the HOSF.
 16. The method according to claim 14, wherein thehandover indication is represented by a REPLACES header field, and thesession information is carried by the REPLACES header field; or, thehandover indication is represented by an address pre-configured as adestination address of the connection establishment request.
 17. Themethod according to claim 14, wherein the connection establishmentrequest is transmitted through an IP Multimedia Subsystem (IMS) networkor a Next Generation Network (NGN) from the HODF to the HOAF-CP; themethod further comprises: network entities in the IMS network or the NGNproviding the handover indication to a charging system to indicatewhether a connection is the second connection to perform chargingreduction.
 18. The method according to claim 14, wherein the sessioninformation of the first connection is obtained by the HOSF during theestablishment of the first connection and sent to the HODF by the HOSF;or, a Handover Control Function (HOCF) is configured on the home networkside of the first user, the session information of the first connectionis obtained by the HOCF during the establishment of the first connectionand sent to the HODF by the HOCF.
 19. The method according to claim 1,wherein the HODF initiates to establish the second connection, the HODFis located in a Circuit Switched (CS) domain, and the HOAF-CP is locatedin an IP Multimedia Subsystem (IMS) domain, the method comprising: theHODF initiating in the CS domain a CS call establishment requestcarrying a called party number; and the CS domain sending the CS callestablishment request to an interworking gateway between the CS domainand the IMS domain; wherein the called party number is at least one of:a Mobile Station Integrated Service Digital Network (MSISDN) number ofthe second user, an E.164 number in a Public User Identity of the seconduser in a Tel-Uniform Resource Identifier (Tel-URI) format, an MSISDNnumber with a prefix, an E.164 number with a prefix, and an E.164 numbercomposed according to an identifier pointing to the HOAF-CP obtained bythe HODF; after receiving the CS call establishment request, theinterworking gateway between the CS domain and the IMS domain convertingthe CS call establishment request into an IMS session establishmentrequest, and sending the IMS session establishment request to theHOAF-CP.
 20. The method according to claim 1, wherein the HOAF-CPinitiates to establish the second connection, and the method comprises:the HOAF-CP obtaining an HODF identifier pointing to the HODF, andinitiating a connection establishment request to the HODF according tothe HODF identifier; after receiving the connection establishmentrequest, the HODF interacting with the HOAF-CP to establish the controlplane second connection with the HOAF-CP, and the user plane secondconnection with the HOAF-CP.
 21. The method according to claim 20,wherein the connection establishment request is sent by the HOAF-CP tothe HODF through an IP Multimedia Subsystem (IMS) network or a NextGeneration Network (NGN), and the connection establishment requestcarries information of the HOSF or a handover indication, the methodfurther comprises: network entities in the IMS network or the NGNproviding the handover indication or the information of the HOSF to acharging system to indicate whether a connection is the secondconnection to perform charging reduction.
 22. The method according toclaim 20, wherein the HODF identifier is sent to the HOAF-CP by theHOSF, or, a Handover Control Function (HOCF) is configured on the homenetwork side of the first user, the HODF identifier is obtained by theHOCF during the establishment of the first connection and sent to theHOAF-CP by the HOCF.
 23. The method according to claim 20, wherein theconnection establishment request carries information of the HOSF; beforethe HODF interacts with the HOAF-CP, the method further comprises: theHODF checking whether the connection establishment request carries theinformation of the HOSF; if the connection establishment request carriesthe information of the HOSF, checking whether the connectionestablishment request is legal according to a pre-configured associationbetween the HOSF and the HODF.
 24. The method according to claim 1,wherein the HODF or the HOAF-CP initiates to establish the secondconnection the method further comprising: the HOAF-CP sending a sessionestablishment request to an interworking gateway, wherein the sessionestablishment request carries a called party number, and the calledparty number is a Mobile Station Integrated Service Digital Network(MSISDN) number of the first user pointing to the HODF or a MobileStation Integrated Service Digital Network (MSISDN) number with aprefix; after receiving the session establishment request, theinterworking gateway converting the session establishment request into aCS call establishment request, and sending the CS call establishmentrequest to the HODF in the CS domain according to the called partynumber; and after receiving the CS call establishment request, the HODFinteracting with the HOAF-CP to establish the control plane secondconnection, and the user plane second connection is established betweenthe HODF and HOAF-UP.
 25. The method according to claim 24, furthercomprising: after the CS domain receiving the CS call establishmentrequest, the CS domain adding an identifier in a Call Detailed Record(CDR) according to the prefix or an indication carried in the CS callestablishment request.
 26. The method according to claim 1, wherein theHOSF and the HODF are respectively corresponding to different IPMultimedia public identities (IMPUs) of a same IP Multimedia privateidentity (IMPI) of the first user; or respectively corresponding todifferent IMPIs of a same IMPU of the first user; or respectivelycorresponding to different IMPUs and IMPIs belonging to a samesubscription of the first user in an IMS network; or associated witheach other through the first user subscription relationship betweendifferent subsystems on an operator layer.
 27. The method according toclaim 1, wherein the HODF initiates to establish the second connectionand the method comprises: the HODF obtaining an identifier pointing tothe HOAF-CP and initiating a connection establishment request accordingto the identifier pointing to the HOAF-CP, wherein the identifierpointing to the HOAF-CP is obtained by the HOSF during the establishmentof the first connection and sent to the HODF by the HOSF, or theidentifier pointing to the HOAF-CP is obtained by the HODF according toan advance configuration of the HODF.
 28. The method according to claim1, wherein the HOSF and the HODF of the first user have different IPMultimedia public identities (IMPUs), and the method comprises: a homenetwork of the first user receiving a session establishment request tothe HOSF of the first user from a third user; the HOAF-CP on the homenetwork side of the first user modifying a destination identifierpointing to the HOSF in the session establishment request to the onepointing to the HODF, and then the home network of the first usersending the session establishment request to the HODF; and the HODFdetermining whether to accept the session establishment request from thethird user.
 29. A system for maintaining session continuity, comprising:a Handover Source Function (HOSF), a Handover Destination Function(HODF), a Handover Anchor Function-Control Plane (HOAF-CP), a HandoverAnchor Function-User Plane (HOAF-UP) and a Handover Detection Function;wherein the HOSF and the HODF are located in a same terminal of a firstuser or different terminals of the first user, the HOAF-CP is located ona home network side of the first user, the HOAF-UP is located on aterminal side of a second user, and the HOAF-CP is a control resource;the HOSF is configured to establish a first connection comprising acontrol plane first connection between the HOSF and the HOAF-CP and auser plane first connection between the HOSF and the HOAF-UP when thefirst user initiates or accepts a session; the HODF is configured toestablish a second connection comprising a control plane secondconnection between the HODF and the HOAF-CP and a user plane secondconnection between the HODF and the HOAF-UP when a handover condition issatisfied; the HOAF-CP is configured to establish the control planefirst connection between the HOSF and the HOAF-CP when the first userinitiates or accepts the session; establish a control plane connectionbetween the terminal side of the second user and the HOAF-CP; establishthe control plane second connection between the HODF and the HOAF-CPwhen the handover condition is satisfied; the HOAF-CP is furtherconfigured to perform control over the control plane first connection,the control plane second connection and the control plane connectionwith the terminal side of the second user, and replace the control planefirst connection between the HOSF and the HOAF-CP with the control planesecond connection between the HODF and the HOAF-CP; the HOAF-UP isconfigured to establish the user plane first connection between the HOSFand the HOAF-UP during the establishment of the control plan plane firstconnection and the control plane connection between the HOAF-CP and theterminal side of the second user; establish the user plane secondconnection between the HODF and the HOAF-UP during the establishment ofthe control plan plane second connection; and replace the user planefirst connection with the user plane second connection; and the HandoverDetection Function is configured to detect whether the handovercondition is satisfied.
 30. The system according to claim 29, whereinthe HOAF-CP is configured to perform control over the establishment ofthe control plane first connection and the control plane connectionbetween the HOAF-CP and the terminal side of the second user, control toestablish the user plane first connection between the HOSF and theHOAF-UP on the terminal side of the second user; and during theestablishment of the control plane second connection, initiate are-negotiation on the control plane connection between the HOAF-CP andthe terminal side of the second user and accomplishing there-negotiation by interacting with the terminal side of the second user,control to establish the user plane second connection between the HODFand the HOAF-UP and indicating the HOAF-UP to replace the user planefirst connection with the user plane second connection, and when mediacapabilities of the HODF and the HOSF are different from each other, addor delete media stream or modify attribute of exchanged media streamaccording to the media capability of the HODF.
 31. The system accordingto claim 29, further comprising: an HOCF on the home network side of thefirst user, configured to control the establishments of the firstconnection and the second connection by sending signal streams betweenthe HOSF and the HOAF-CP, or between the HODF and the HOAF-CP.
 32. Thesystem according to claim 31, wherein the Handover Detection Function isa logic unit in the HOCF, and the HOCF is further configured to send tothe HODF or the HOAF-CP, or send a handover indication to the HODFthrough the HOSF when detecting that the handover condition issatisfied.
 33. The system according to claim 31, wherein the HandoverDetection Function is further configured to send a handover requestmessage to the HOCF when detecting that the handover condition issatisfied, and send a handover indication to the HODF or the HOAF-CPafter receiving an authorization pass notification from the HOCF; andthe HOCF is further configured to perform an authorization whenreceiving the handover request message, and send the authorization passnotification to the Handover Detection Function after authorizationsucceeds.
 34. The system according to claim 32, wherein the HODF isfurther configured to initiate a second connection establishment requestto the HOAF-CP after receiving the handover indication.
 35. The systemaccording to claim 32, wherein the HOAF-CP is further configured toinitiate a second connection establishment request to the HODF afterreceiving the handover indication.
 36. The system according to claim 29,wherein the Handover Detection Function is further configured to send ahandover indication to the HODF or the HOAF-CP when detecting that thehandover condition is satisfied.
 37. The system according to claim 29,wherein the Handover Detection Function is a logic unit in the HODF, theHODF is further configured to initiate a second connection establishmentrequest to the HOAF-CP when-the handover condition is satisfied.
 38. Thesystem according to claim 29, wherein when receiving a connectionestablishment request from a third user to the HOSF of the first user,the HOCF or the HOAF-CP located on the home network side of the firstuser is configured to send a new session from the third user to the HODFof the first user.
 39. A system for maintaining session continuity,comprising: a handover Source Function (HOSF), a Handover DestinationFunction (HODF), a Handover Anchor Function-Control Plane (HOAF-CP), aHandover Anchor Function-User Plane (HOAF-UP) and a Handover DetectionFunction; wherein the HOSF and the HODF are located in a same terminalor different terminals of a first user, the HOAF-CP and HOAF-UP arelocated on a home network side of the first user, the HOAF-CP is controlresource, the HOAF-UP is media resource; the HOSF is configured toestablish a first connection comprising a control plane first connectionbetween the HOSF and the HOAF-CP and a user plane first connectionbetween the HOSF and the HOAF-UP when the first user initiates oraccepts a session; the HODF is configured to establish a secondconnection comprising a control plane second connection between the HODFand the HOAF-CP and a user plane second connection between the HODF andthe HOAF-UP when a handover condition is satisfied; the HOAF-CP isconfigured to establish the control plane first connection between theHOSF and the HOAF-CP when the first user initiates or accepts thesession; establish a control plane connection between a terminal side ofa second user and the HOAF-CP; establish the control plane secondconnection between the HODF and the HOAF-CP when the handover conditionis satisfied; the HOAF-CP is further configured to perform control overthe control plane first connection, the control plane second connectionand the control plane connection between the terminal side of the seconduser and the HOAF-CP, replace the control plane first connection betweenthe HOSF and the HOAF-CP with the control plane second connectionbetween the HODF and the HOAF-CP; the HOAF-UP is configured to establishthe user plane first connection between the HOSF and the HOAF-UP;establish a user plane connection between the terminal side of thesecond user and the HOAF-UP, and connect the user plane connectionbetween the HOAF-UP and the terminal side of the second user with theuser plane first connection; establish the user plane second connectionbetween the HODF and the HOAF-UP; and connect the user plane connectionbetween the HOAF-UP and the terminal side of the second user with theuser plane second connection instead of the user plane first connection;and the Handover Detection Function is configured to detect whether thehandover condition is satisfied.
 40. The system according to claim 39,wherein the HOAF-CP is configured to: perform control over theestablishments of the control plane first connection and the controlplane connection between the terminal side of the second user and theHOAF-CP, control to establish the user plane first connection betweenthe HOAF-UP and the HOSF and the user plane connection between theterminal side of the second user and the HOAF-UP, and control theHOAF-UP to connect the user plane first connection and the user planesecond connection; and during the establishment of the control planesecond connection between the HOAF-CP and the HODF, control to establishthe user plane second connection between the HOAF-UP and the HODF,control the HOAF-UP to connect the user plane connection between theHOAF-UP and the terminal side of the second user, with the user planesecond connection instead of the user plane first connection, and whenmedia capabilities of the HODF and the HOSF are different from eachother, during the establishment of the control plane second connection,initiate a re-negotiation on the control plane connection with theterminal side of the second user and accomplishing the re-negotiation byinteracting with the terminal side of the second user, add or deletemedia stream or modifying attribute of exchanged media stream betweenthe HOAF-UP and the terminal side of the second user according to themedia capability of the HODF.
 41. A method for maintaining sessioncontinuity, comprising: triggering a Handover Anchor Function-ControlPlane (HOAF-CP) on a home network side of a first user, and establishinga first connection comprising a control plane first connection between aHandover Source Function (HOSF) of the first user and the HOAF-CP and auser plane first connection between the HOSF and a Handover AnchorFunction-User Plane (HOAF-UP) on the home network side of the firstuser, establishing a control plane connection between the HOAF-CP and aterminal side of a second user and a user plane connection between theHOAF-UP and the terminal side of the second user, and establishing asession between the first user and the second user through the controlplane first connection, the control plane connection between the HOAF-CPand the terminal side of the second user, the user plane firstconnection, and the user plane connection between the HOAF-UP and theterminal side of the second user; determining that a handover conditionis satisfied, establishing a second connection, wherein the secondconnection comprises a control plane second connection between aHandover Destination Function (HODF) of the first user and the HOAF-CPand a user plane second connection between the HODF and the HOAF-UP;continuing the session between the first user and the second userthrough the control plane second connection between the HODF of thefirst user and the HOAF-CP and the control plane connection between theHOAF-CP and the terminal side of the second user, and the user planesecond connection between the HODF of the first user and the HOAF-UP andthe user plane connection between the HOAF-UP and the terminal side ofthe second user.
 42. A method for maintaining session continuity,comprising: establishing, by a Handover Anchor Function-Control Plane(HOAF-CP) of an IP multimedia sub-system (IMS) on a home network side ofa first user, a control plane first connection and a control plane thirdconnection, wherein the control plane first connection is between a userterminal of the first user and the HOAF-CP, wherein the control planethird connection is between the HOAF-CP and a terminal side of a seconduser; establishing a user plane first connection between the userterminal and a Handover Anchor Function-User Plane (HOAF-UP) of the IMS,wherein the control plane first connection, the control plane thirdconnection and the user plane first connection achieves a sessionbetween the first user and the second user; when a handover condition issatisfied, establishing, by the HOAF-CP, a control plane secondconnection between the user terminal of the first user and the HOAF-CP,and establishing a user plane second connection between the userterminal and the HOAF-UP; and replacing, by the HOAF-CP, the controlplane first connection with the control plane second connection, andreplacing the user plane first connection with the user plane secondconnection, and wherein the control plane second connection, the controlplane third connection and the user plane second connection achieves thesession after the handover.
 43. The method according to claim 42,further comprising: receiving, by the HOAF-CP, a connectionestablishment request from the user terminal after the handovercondition is satisfied.
 44. The method according to claim 42, furthercomprising: receiving, by the HOAF-CP, a handover indication from theuser terminal after the handover condition is satisfied, sending, by theHOAF-CP, a connection establishment request to the user terminal. 45.The method according to claim 42, further comprising: receiving, by theHOAF-CP, a handover indication from the user terminal after the handovercondition is satisfied; and sending a connection establishment requestto the user terminal.
 46. The method according to claim 42, wherein thestep of establishing the control plane second connection comprises:receiving, by the HOAF-CP, a connection establishment request from theuser terminal, wherein the connection establishment request carriessession information of the the control plane first connection and ahandover indication; after receiving the connection establishmentrequest, the HOAF-CP interacting with the user terminal to establish thecontrol plane second connection between the user terminal and theHOAF-CP.
 47. The method according to claim 42, wherein the HOAF-UP islocated on a terminal side of the second user.
 48. A system formaintaining session continuity, comprising a Handover AnchorFunction-Control Plane (HOAF-CP) on a home network side of a first user,wherein the HOAF-CP is configured to communicate with a user terminal ofthe first user, wherein the HOAF-CP is further configured to: establisha control plane first connection and a control plane third connection,wherein the control plane first connection is between the user terminaland the HOAF-CP, and wherein the control plane third connection isbetween the HOAF-CP and a terminal side of a second user; control toestablish a user plane first connection between the user terminal and aHandover Anchor Function-User Plane (HOAF-UP), wherein the control planefirst connection, the control plane third connection and the user planefirst connection achieves a session between the first user and thesecond user; when a handover condition is satisfied, establish a controlplane second connection between the user terminal and the HOAF-CP, andcontrol to establish a user plane second connection between the userterminal and the HOAF-UP; and replace the control plane first connectionwith the control plane second connection, and indicate the HOAF-UP toreplace the user plane first connection with the user plane secondconnection, and wherein the control plane second connection, the controlplane third connection and the user plane second connection achieves thesession after the handover.
 49. A method for maintaining sessioncontinuity, comprising: establishing a control plane first connectionand a control plane third connection, wherein the control plane firstconnection is between a user terminal of the first user and a HandoverAnchor Function-Control Plane (HOAF-CP) of an IP multimedia sub-system(IMS) on a home network side of the first user, wherein the controlplane third connection is between the HOAF-CP and a terminal side of asecond user; establishing a user plane first connection between the userterminal and a Handover Anchor Function-User Plane (HOAF-UP) of the IMSand a user plane third connection between the HOAF-UP and the terminalside of the second user, wherein the control plane first connection, thecontrol plane third connection, the user plane first connection and theuser plane third connection achieves a session between the first userand the second user; when a handover condition is satisfied,establishing, by the HOAF-CP, a control plane second connection betweenthe user terminal of the first user and the HOAF-CP, and establishing auser plane second connection between the user terminal and the HOAF-UP;and replacing, by the HOAF-CP, the control plane first connection withthe control plane second connection, and replacing the user plane firstconnection with the user plane second connection, and wherein thecontrol plane second connection, the control plane third connection, theuser plane second connection and the user plane third connectionachieves the session after the handover.
 50. A system for maintainingsession continuity, comprising a Handover Anchor Function-Control Plane(HOAF-CP) of an IP multimedia sub-system (IMS) on a home network side ofa first user, wherein the HOAF-CP is configured to communicate with auser terminal of the first user and a user terminal of the first user,wherein the HOAF-CP is further configured to: establish a control planefirst connection and a control plane third connection, wherein thecontrol plane first connection is between the user terminal and theHOAF-CP, and wherein the control plane third connection is between theHOAF-CP and a terminal side of a second user; control to establish auser plane first connection between the user terminal and a HandoverAnchor Function-User Plane (HOAF-UP) of the IMS and a user plane thirdconnection between the HOAF-UP and the terminal side of the second user,wherein the control plane first connection, the control plane thirdconnection, the user plane first connection and the user plane thirdconnection achieves a session between the first user and the seconduser; when a handover condition is satisfied, establish a control planesecond connection between the user terminal and the HOAF-CP, and controlto establish a user plane second connection between the user terminaland the HOAF-UP; and replace the control plane first connection with thecontrol plane second connection, and indicate the HOAF-UP to replace theuser plane first connection with the user plane second connection, andwherein the control plane second connection, the control plane thirdconnection, the user plane second connection and the user plane thirdconnection achieves the session after the handover.
 51. A system formaintaining session continuity, comprising a Handover AnchorFunction-Control Plane (HOAF-CP) of an IP multimedia sub-system (IMS) ona home network side of a first user, a Handover Anchor Function-UserPlane (HOAF-UP) of the IMS, wherein the HOAF-CP is configured to:establish a control plane first connection and a control plane thirdconnection, wherein the control plane first connection is between a userterminal of the first user and the HOAF-CP, and wherein the controlplane third connection is between the HOAF-CP and a terminal side of asecond user; control to establish a user plane first connection betweenthe user terminal and a Handover Anchor Function-User Plane (HOAF-UP)and a user plane third connection between the HOAF-UP and the terminalside of the second user, wherein the control plane first connection, thecontrol plane third connection, the user plane first connection and theuser plane third connection achieves a session between the first userand the second user; when a handover condition is satisfied, establish acontrol plane second connection between the user terminal of the firstuser and the HOAF-CP, and control to establish a user plane secondconnection between the user terminal and the HOAF-UP; and replace thecontrol plane first connection with the control plane second connection,and indicate the HOAF-UP to replace the user plane first connection withthe user plane second connection, and wherein the control plane secondconnection, the control plane third connection, the user plane secondconnection and the user plane third connection achieves the sessionafter the handover; wherein the HOAF-UP is configured to establish theuser plane first connection, the user plane second connection and theuser plane third connection, and replace the user plane first connectionwith the user plane second connection.
 52. A method for maintainingsession continuity, comprising: establishing a first connection, whereinthe first connection comprises a control plane first connection betweena user terminal of the first user and a Handover Anchor Function-ControlPlane (HOAF-CP) on a home network side of the first user and a userplane first connection between the user terminal and a Handover AnchorFunction-User Plane (HOAF-UP) of a second user; establishing a controlplane connection between the HOAF-CP and a terminal side of the seconduser, wherein the control plane first connection, the control planethird connection, and the user plane first connection achieve a sessionbetween the first user and the second user; determining that a handovercondition is satisfied, establishing a second connection, wherein thesecond connection comprises a control plane second connection betweenthe user terminal of the first user and the HOAF-CP and a user planesecond connection between the user terminal and the HOAF-UP; and whereinthe user plane second connection between the user terminal and theHOAF-UP, the control plane second connection between the user terminaland the HOAF-CP, and the control plane connection, which is alreadyestablished between the HOAF-CP and the terminal side of the second userbefore the handover condition is satisfied, achieves the session afterthe handover.
 53. A method for maintaining session continuity,comprising: establishing a first connection comprising a control planefirst connection between a user terminal of the first user and aHandover Anchor Function-Control Plane (HOAF-CP) of an IP multimediasub-system (IMS) on a home network side of the first user and a userplane first connection between the user terminal and a Handover AnchorFunction-User Plane (HOAF-UP) of the IMS on the home network side of thefirst user; establishing a control plane connection between the HOAF-CPand a terminal side of a second user and a user plane connection betweenthe HOAF-UP and the terminal side of the second user, wherein thecontrol plane first connection, the control plane connection, the userplane connection and the user plane first connection achieve a sessionbetween the first user and the second user; determining that a handovercondition is satisfied, establishing a second connection, wherein thesecond connection comprises a control plane second connection betweenthe user terminal of the first user and the HOAF-CP and a user planesecond connection between the user terminal and the HOAF-UP; wherein thecontrol plane second connection between the user terminal of the firstuser and the HOAF-CP, the control plane connection between the HOAF-CPand the terminal side of the second user, the user plane secondconnection between the user terminal of the first user and the HOAF-UP,and the user plane connection between the HOAF-UP and the terminal sideof the second user achieve the session after the handover.
 54. A methodfor maintaining session continuity, comprising: establishing a firstconnection comprising a control plane first connection between a userterminal of a first user and a Handover Anchor Function-Control Plane(HOAF-CP) of an IP multimedia sub-system (IMS) on a home network side ofthe first user and a user plane first connection between the userterminal and a Handover Anchor Function-User Plane (HOAF-UP) of the IMSon the home network side of the first user; establishing a control planeconnection between the HOAF-CP and a terminal side of a second user anda user plane connection between the HOAF-UP and the terminal side of thesecond user, and establishing a session between the first user and thesecond user through the control plane first connection, the controlplane connection between the HOAF-CP and the terminal side of the seconduser, the user plane first connection and the user plane connectionbetween the HOAF-UP and the terminal side of the second user; when ahandover condition is satisfied, establishing a second connection,wherein the second connection comprises a control plane secondconnection between the user terminal of the first user and the HOAF-CPand a user plane second connection between the user terminal and theHOAF-UP; continuing the session between the first user and the seconduser through the control plane second connection, the control planeconnection between the HOAF-CP and the terminal side of the second user,the user plane second connection and the user plane connection betweenthe HOAF-UP and the terminal side of the second user.
 55. The method ofclaim 56, wherein the control plane first connection and the user planefirst connection are connections of Packet Switched (PS)domain, thecontrol plane second connection and the user plane second connection areconnections of Circuit Switched (CS) domain.
 56. A method formaintaining session continuity, comprising: establishing a control planefirst connection between a user terminal of a first user and a HandoverAnchor Function-Control Plane (HOAF-CP) of an IP multimedia sub-system(IMS) on a home network side of the first user and a user plane firstconnection between the user terminal and a Handover Anchor Function-UserPlane (HOAF-UP) of the IMS on the home network side of the first user;establishing a control plane third connection between the HOAF-CP and aterminal side of a second user and a user plane third connection betweenthe HOAF-UP and the terminal side of the second user, and establishing asession between the first user and the second user through the controlplane first connection, the control plane third connection between theHOAF-CP and the terminal side of the second user, the user plane firstconnection and the user plane third connection between the HOAF-UP andthe terminal side of the second user; when a handover condition issatisfied, establishing a control plane second connection between theuser terminal of the first user and the HOAF-CP and a user plane secondconnection between the user terminal and the HOAF-UP; wherein thecontrol plane second connection between the user terminal of the firstuser and the HOAF-CP, the control plane third connection between theHOAF-CP and the terminal side of the second user, the user plane secondconnection between the user terminal of the first user and the HOAF-UP,and the user plane third connection between the HOAF-UP and the terminalside of the second user achieve the session after the handover.
 57. Themethod of claim 56, wherein the control plane first connection and theuser plane first connection are connections of Packet Switched(PS)domain, the control plane second connection and the user planesecond connection are connections of Circuit Switched (CS) domain. 58.The method of claim 42, wherein the HOAF-CP is a server of an IPmultimedia subsystem (IMS).
 59. The system of claim 48, wherein theHOAF-CP is a server of an IP multimedia subsystem (IMS).
 60. The methodof claim 49, wherein the control plane first connection and the userplane first connection are connections of Packet Switched (PS)domain,the control plane second connection and the user plane second connectionare connections of Circuit Switched (CS) domain.
 61. The system of claim50, wherein the control plane first connection and the user plane firstconnection are connections of Packet Switched (PS) domain, the controlplane second connection and the user plane second connection areconnections of Circuit Switched (CS) domain.
 62. The system of claim 51,wherein the control plane first connection and the user plane firstconnection are connections of Packet Switched (PS) domain, the controlplane second connection and the user plane second connection areconnections of Circuit Switched (CS) domain.
 63. The method of claim 52,wherein the HOAF-CP is a server of an IP multimedia subsystem (IMS). 64.The method of claim 53, wherein the control plane first connection andthe user plane first connection are connections of Packet Switched (PS)domain, the control plane second connection and the user plane secondconnection are connections of Circuit Switched (CS) domain.